[ovs-dev] [eviction 03/12] meta-flow: New "subfield" data structure.

Ethan Jackson ethan at nicira.com
Mon Jan 23 21:13:03 UTC 2012


In meta-flow.c

In two places you mention ofp_mkerr() which no longer exists.

In mf_parse_subfield__() you modify 's' even if there's an error which
would be fine if the comments didn't say that' impossible.

Otherwise looks good.

Ethan

On Fri, Jan 13, 2012 at 16:43, Ben Pfaff <blp at nicira.com> wrote:
> Until now, parts of a field have been dealt with in a fairly ad-hoc way.
> struct mf_subfield and the supporting functions added by this commit make
> their use more systematic.
> ---
>  lib/autopath.c         |   35 +++--
>  lib/bundle.c           |   39 +++---
>  lib/learn.c            |  194 ++++++++++++----------------
>  lib/meta-flow.c        |  333 ++++++++++++++++++++++++++++++++++++++----------
>  lib/meta-flow.h        |   26 ++++-
>  lib/multipath.c        |   40 +++---
>  lib/nx-match.c         |  293 +++++++++++++-----------------------------
>  lib/nx-match.h         |   24 +---
>  lib/ofp-parse.c        |    9 +-
>  lib/ofp-print.c        |   12 +-
>  lib/ofp-util.c         |    6 +-
>  ofproto/ofproto-dpif.c |    5 +-
>  12 files changed, 551 insertions(+), 465 deletions(-)
>
> diff --git a/lib/autopath.c b/lib/autopath.c
> index 321b106..b8ba3c1 100644
> --- a/lib/autopath.c
> +++ b/lib/autopath.c
> @@ -22,6 +22,7 @@
>  #include <stdlib.h>
>
>  #include "flow.h"
> +#include "meta-flow.h"
>  #include "nx-match.h"
>  #include "ofp-errors.h"
>  #include "ofp-util.h"
> @@ -36,23 +37,26 @@ void
>  autopath_execute(const struct nx_action_autopath *ap, struct flow *flow,
>                  uint16_t ofp_port)
>  {
> -    nxm_reg_load(ap->dst, ap->ofs_nbits, ofp_port, flow);
> +    struct mf_subfield dst;
> +
> +    nxm_decode(&dst, ap->dst, ap->ofs_nbits);
> +    mf_set_subfield_value(&dst, ofp_port, flow);
>  }
>
>  void
>  autopath_parse(struct nx_action_autopath *ap, const char *s_)
>  {
>     char *s;
> -    uint32_t reg;
> -    int id_int, ofs, n_bits;
> -    char *id_str, *dst, *save_ptr;
> +    char *id_str, *dst_s, *save_ptr;
> +    struct mf_subfield dst;
> +    int id_int;
>
>     s = xstrdup(s_);
>     save_ptr = NULL;
>     id_str = strtok_r(s, ", ", &save_ptr);
> -    dst = strtok_r(NULL, ", ", &save_ptr);
> +    dst_s = strtok_r(NULL, ", ", &save_ptr);
>
> -    if (!dst) {
> +    if (!dst_s) {
>         ovs_fatal(0, "%s: not enough arguments to autopath action", s_);
>     }
>
> @@ -62,16 +66,17 @@ autopath_parse(struct nx_action_autopath *ap, const char *s_)
>                   "1 to %"PRIu32, s_, id_int, UINT32_MAX);
>     }
>
> -    nxm_parse_field_bits(dst, &reg, &ofs, &n_bits);
> -    if (n_bits < 16) {
> +    mf_parse_subfield(&dst, dst_s);
> +    if (dst.n_bits < 16) {
>         ovs_fatal(0, "%s: %d-bit destination field has %u possible values, "
> -                  "less than required 65536", s_, n_bits, 1u << n_bits);
> +                  "less than required 65536",
> +                  s_, dst.n_bits, 1u << dst.n_bits);
>     }
>
>     ofputil_init_NXAST_AUTOPATH(ap);
>     ap->id = htonl(id_int);
> -    ap->ofs_nbits = nxm_encode_ofs_nbits(ofs, n_bits);
> -    ap->dst = htonl(reg);
> +    ap->ofs_nbits = nxm_encode_ofs_nbits(dst.ofs, dst.n_bits);
> +    ap->dst = htonl(dst.field->nxm_header);
>
>     free(s);
>  }
> @@ -79,14 +84,14 @@ autopath_parse(struct nx_action_autopath *ap, const char *s_)
>  enum ofperr
>  autopath_check(const struct nx_action_autopath *ap, const struct flow *flow)
>  {
> -    int n_bits = nxm_decode_n_bits(ap->ofs_nbits);
> -    int ofs = nxm_decode_ofs(ap->ofs_nbits);
> +    struct mf_subfield dst;
>
> -    if (n_bits < 16) {
> +    nxm_decode(&dst, ap->dst, ap->ofs_nbits);
> +    if (dst.n_bits < 16) {
>         VLOG_WARN("at least 16 bit destination is required for autopath "
>                   "action.");
>         return OFPERR_OFPBAC_BAD_ARGUMENT;
>     }
>
> -    return nxm_dst_check(ap->dst, ofs, n_bits, flow);
> +    return mf_check_dst(&dst, flow);
>  }
> diff --git a/lib/bundle.c b/lib/bundle.c
> index 8e98fb5..733d79a 100644
> --- a/lib/bundle.c
> +++ b/lib/bundle.c
> @@ -22,6 +22,7 @@
>
>  #include "dynamic-string.h"
>  #include "multipath.h"
> +#include "meta-flow.h"
>  #include "nx-match.h"
>  #include "ofpbuf.h"
>  #include "ofp-errors.h"
> @@ -94,8 +95,11 @@ bundle_execute_load(const struct nx_action_bundle *nab, struct flow *flow,
>                     bool (*slave_enabled)(uint16_t ofp_port, void *aux),
>                     void *aux)
>  {
> -    nxm_reg_load(nab->dst, nab->ofs_nbits,
> -                 bundle_execute(nab, flow, slave_enabled, aux), flow);
> +    struct mf_subfield dst;
> +
> +    nxm_decode(&dst, nab->dst, nab->ofs_nbits);
> +    mf_set_subfield_value(&dst, bundle_execute(nab, flow, slave_enabled, aux),
> +                          flow);
>  }
>
>  /* Checks that 'nab' specifies a bundle action which is supported by this
> @@ -146,15 +150,15 @@ bundle_check(const struct nx_action_bundle *nab, int max_ports,
>     }
>
>     if (subtype == NXAST_BUNDLE_LOAD) {
> -        int ofs = nxm_decode_ofs(nab->ofs_nbits);
> -        int n_bits = nxm_decode_n_bits(nab->ofs_nbits);
> +        struct mf_subfield dst;
>
> -        if (n_bits < 16) {
> +        nxm_decode(&dst, nab->dst, nab->ofs_nbits);
> +        if (dst.n_bits < 16) {
>             VLOG_WARN_RL(&rl, "bundle_load action requires at least 16 bit "
>                          "destination.");
>             error = OFPERR_OFPBAC_BAD_ARGUMENT;
>         } else if (!error) {
> -            error = nxm_dst_check(nab->dst, ofs, n_bits, flow);
> +            error = mf_check_dst(&dst, flow);
>         }
>     }
>
> @@ -192,7 +196,7 @@ bundle_check(const struct nx_action_bundle *nab, int max_ports,
>  static void
>  bundle_parse__(struct ofpbuf *b, const char *s, char **save_ptr,
>                const char *fields, const char *basis, const char *algorithm,
> -               const char *slave_type, const char *dst,
> +               const char *slave_type, const char *dst_s,
>                const char *slave_delim)
>  {
>     enum ofputil_action_code code;
> @@ -208,7 +212,7 @@ bundle_parse__(struct ofpbuf *b, const char *s, char **save_ptr,
>                    s, slave_delim);
>     }
>
> -    code = dst ? OFPUTIL_NXAST_BUNDLE_LOAD : OFPUTIL_NXAST_BUNDLE;
> +    code = dst_s ? OFPUTIL_NXAST_BUNDLE_LOAD : OFPUTIL_NXAST_BUNDLE;
>     b->l2 = ofputil_put_action(code, b);
>
>     n_slaves = 0;
> @@ -259,14 +263,12 @@ bundle_parse__(struct ofpbuf *b, const char *s, char **save_ptr,
>         ovs_fatal(0, "%s: unknown slave_type `%s'", s, slave_type);
>     }
>
> -    if (dst) {
> -        uint32_t reg;
> -        int ofs, n_bits;
> -
> -        nxm_parse_field_bits(dst, &reg, &ofs, &n_bits);
> +    if (dst_s) {
> +        struct mf_subfield dst;
>
> -        nab->dst = htonl(reg);
> -        nab->ofs_nbits = nxm_encode_ofs_nbits(ofs, n_bits);
> +        mf_parse_subfield(&dst, dst_s);
> +        nab->dst = htonl(dst.field->nxm_header);
> +        nab->ofs_nbits = nxm_encode_ofs_nbits(dst.ofs, dst.n_bits);
>     }
>
>     b->l2 = NULL;
> @@ -359,9 +361,10 @@ bundle_format(const struct nx_action_bundle *nab, struct ds *s)
>                   ntohs(nab->basis), algorithm, slave_type);
>
>     if (nab->subtype == htons(NXAST_BUNDLE_LOAD)) {
> -        nxm_format_field_bits(s, ntohl(nab->dst),
> -                              nxm_decode_ofs(nab->ofs_nbits),
> -                              nxm_decode_n_bits(nab->ofs_nbits));
> +        struct mf_subfield dst;
> +
> +        nxm_decode(&dst, nab->dst, nab->ofs_nbits);
> +        mf_format_subfield(&dst, s);
>         ds_put_cstr(s, ",");
>     }
>
> diff --git a/lib/learn.c b/lib/learn.c
> index 241f3d1..1676462 100644
> --- a/lib/learn.c
> +++ b/lib/learn.c
> @@ -1,5 +1,5 @@
>  /*
> - * Copyright (c) 2011 Nicira Networks.
> + * Copyright (c) 2011, 2012 Nicira Networks.
>  *
>  * Licensed under the Apache License, Version 2.0 (the "License");
>  * you may not use this file except in compliance with the License.
> @@ -59,6 +59,14 @@ get_bits(int n_bits, const void **p)
>     return value;
>  }
>
> +static void
> +get_subfield(int n_bits, const void **p, struct mf_subfield *sf)
> +{
> +    sf->field = mf_from_nxm_header(ntohl(get_be32(p)));
> +    sf->ofs = ntohs(get_be16(p));
> +    sf->n_bits = n_bits;
> +}
> +
>  static unsigned int
>  learn_min_len(uint16_t header)
>  {
> @@ -143,10 +151,10 @@ learn_check(const struct nx_action_learn *learn, const struct flow *flow)
>
>         /* Check the source. */
>         if (src_type == NX_LEARN_SRC_FIELD) {
> -            ovs_be32 src_field = get_be32(&p);
> -            int src_ofs = ntohs(get_be16(&p));
> +            struct mf_subfield src;
>
> -            error = nxm_src_check(src_field, src_ofs, n_bits, flow);
> +            get_subfield(n_bits, &p, &src);
> +            error = mf_check_src(&src, flow);
>             if (error) {
>                 return error;
>             }
> @@ -157,20 +165,19 @@ learn_check(const struct nx_action_learn *learn, const struct flow *flow)
>
>         /* Check the destination. */
>         if (dst_type == NX_LEARN_DST_MATCH || dst_type == NX_LEARN_DST_LOAD) {
> -            ovs_be32 dst_field = get_be32(&p);
> -            int dst_ofs = ntohs(get_be16(&p));
> +            struct mf_subfield dst;
>
> +            get_subfield(n_bits, &p, &dst);
>             error = (dst_type == NX_LEARN_DST_LOAD
> -                     ? nxm_dst_check(dst_field, dst_ofs, n_bits, &rule.flow)
> -                     : nxm_src_check(dst_field, dst_ofs, n_bits, &rule.flow));
> +                     ? mf_check_dst(&dst, &rule.flow)
> +                     : mf_check_src(&dst, &rule.flow));
>             if (error) {
>                 return error;
>             }
>
>             if (dst_type == NX_LEARN_DST_MATCH
>                 && src_type == NX_LEARN_SRC_IMMEDIATE) {
> -                mf_set_subfield(mf_from_nxm_header(ntohl(dst_field)), value,
> -                                dst_ofs, n_bits, &rule);
> +                mf_set_subfield(&dst, value, &rule);
>             }
>         }
>     }
> @@ -210,38 +217,32 @@ learn_execute(const struct nx_action_learn *learn, const struct flow *flow,
>         uint64_t value;
>
>         struct nx_action_reg_load *load;
> -        ovs_be32 dst_field;
> -        int dst_ofs;
> +        struct mf_subfield dst;
>
>         if (!header) {
>             break;
>         }
>
>         if (src_type == NX_LEARN_SRC_FIELD) {
> -            ovs_be32 src_field = get_be32(&p);
> -            int src_ofs = ntohs(get_be16(&p));
> +            struct mf_subfield src;
>
> -            value = nxm_read_field_bits(src_field,
> -                                        nxm_encode_ofs_nbits(src_ofs, n_bits),
> -                                        flow);
> +            get_subfield(n_bits, &p, &src);
> +            value = mf_get_subfield(&src, flow);
>         } else {
>             value = get_bits(n_bits, &p);
>         }
>
>         switch (dst_type) {
>         case NX_LEARN_DST_MATCH:
> -            dst_field = get_be32(&p);
> -            dst_ofs = ntohs(get_be16(&p));
> -            mf_set_subfield(mf_from_nxm_header(ntohl(dst_field)), value,
> -                            dst_ofs, n_bits, &fm->cr);
> +            get_subfield(n_bits, &p, &dst);
> +            mf_set_subfield(&dst, value, &fm->cr);
>             break;
>
>         case NX_LEARN_DST_LOAD:
> -            dst_field = get_be32(&p);
> -            dst_ofs = ntohs(get_be16(&p));
> +            get_subfield(n_bits, &p, &dst);
>             load = ofputil_put_NXAST_REG_LOAD(&actions);
> -            load->ofs_nbits = nxm_encode_ofs_nbits(dst_ofs, n_bits);
> -            load->dst = dst_field;
> +            load->ofs_nbits = nxm_encode_ofs_nbits(dst.ofs, dst.n_bits);
> +            load->dst = htonl(dst.field->nxm_header);
>             load->value = htonll(value);
>             break;
>
> @@ -283,19 +284,18 @@ struct learn_spec {
>     int n_bits;
>
>     int src_type;
> -    const struct mf_field *src;
> -    int src_ofs;
> +    struct mf_subfield src;
>     uint8_t src_imm[sizeof(union mf_value)];
>
>     int dst_type;
> -    const struct mf_field *dst;
> -    int dst_ofs;
> +    struct mf_subfield dst;
>  };
>
>  static void
>  learn_parse_spec(const char *orig, char *name, char *value,
>                  struct learn_spec *spec)
>  {
> +    memset(spec, 0, sizeof *spec);
>     if (mf_from_name(name)) {
>         const struct mf_field *dst = mf_from_name(name);
>         union mf_value imm;
> @@ -308,49 +308,36 @@ learn_parse_spec(const char *orig, char *name, char *value,
>
>         spec->n_bits = dst->n_bits;
>         spec->src_type = NX_LEARN_SRC_IMMEDIATE;
> -        spec->src = NULL;
> -        spec->src_ofs = 0;
>         memcpy(spec->src_imm, &imm, dst->n_bytes);
>         spec->dst_type = NX_LEARN_DST_MATCH;
> -        spec->dst = dst;
> -        spec->dst_ofs = 0;
> +        spec->dst.field = dst;
> +        spec->dst.ofs = 0;
> +        spec->dst.n_bits = dst->n_bits;
>     } else if (strchr(name, '[')) {
> -        uint32_t src_header, dst_header;
> -        int src_ofs, dst_ofs;
> -        int n_bits;
> -
>         /* Parse destination and check prerequisites. */
> -        if (nxm_parse_field_bits(name, &dst_header, &dst_ofs,
> -                                 &n_bits)[0] != '\0') {
> +        if (mf_parse_subfield(&spec->dst, name)[0] != '\0') {
>             ovs_fatal(0, "%s: syntax error after NXM field name `%s'",
>                       orig, name);
>         }
>
>         /* Parse source and check prerequisites. */
>         if (value[0] != '\0') {
> -            int src_nbits;
> -
> -            if (nxm_parse_field_bits(value, &src_header, &src_ofs,
> -                                     &src_nbits)[0] != '\0') {
> +            if (mf_parse_subfield(&spec->src, value)[0] != '\0') {
>                 ovs_fatal(0, "%s: syntax error after NXM field name `%s'",
>                           orig, value);
>             }
> -            if (src_nbits != n_bits) {
> -                ovs_fatal(0, "%s: bit widths of %s (%d) and %s (%d) differ",
> -                          orig, name, dst_header, value, dst_header);
> +            if (spec->src.n_bits != spec->dst.n_bits) {
> +                ovs_fatal(0, "%s: bit widths of %s (%u) and %s (%u) differ",
> +                          orig, name, spec->src.n_bits, value,
> +                          spec->dst.n_bits);
>             }
>         } else {
> -            src_header = dst_header;
> -            src_ofs = dst_ofs;
> +            spec->src = spec->dst;
>         }
>
> -        spec->n_bits = n_bits;
> +        spec->n_bits = spec->src.n_bits;
>         spec->src_type = NX_LEARN_SRC_FIELD;
> -        spec->src = mf_from_nxm_header(src_header);
> -        spec->src_ofs = src_ofs;
>         spec->dst_type = NX_LEARN_DST_MATCH;
> -        spec->dst = mf_from_nxm_header(dst_header);
> -        spec->dst_ofs = 0;
>     } else if (!strcmp(name, "load")) {
>         if (value[strcspn(value, "[-")] == '-') {
>             struct nx_action_reg_load load;
> @@ -365,14 +352,11 @@ learn_parse_spec(const char *orig, char *name, char *value,
>
>             spec->n_bits = nbits;
>             spec->src_type = NX_LEARN_SRC_IMMEDIATE;
> -            spec->src = NULL;
> -            spec->src_ofs = 0;
>             for (i = 0; i < imm_bytes; i++) {
>                 spec->src_imm[i] = imm >> ((imm_bytes - i - 1) * 8);
>             }
>             spec->dst_type = NX_LEARN_DST_LOAD;
> -            spec->dst = mf_from_nxm_header(ntohl(load.dst));
> -            spec->dst_ofs = nxm_decode_ofs(load.ofs_nbits);
> +            nxm_decode(&spec->dst, load.dst, load.ofs_nbits);
>         } else {
>             struct nx_action_reg_move move;
>
> @@ -380,28 +364,21 @@ learn_parse_spec(const char *orig, char *name, char *value,
>
>             spec->n_bits = ntohs(move.n_bits);
>             spec->src_type = NX_LEARN_SRC_FIELD;
> -            spec->src = mf_from_nxm_header(ntohl(move.src));
> -            spec->src_ofs = ntohs(move.src_ofs);
> +            nxm_decode_discrete(&spec->src,
> +                                move.src, move.src_ofs, move.n_bits);
>             spec->dst_type = NX_LEARN_DST_LOAD;
> -            spec->dst = mf_from_nxm_header(ntohl(move.dst));
> -            spec->dst_ofs = ntohs(move.dst_ofs);
> +            nxm_decode_discrete(&spec->dst,
> +                                move.dst, move.dst_ofs, move.n_bits);
>         }
>     } else if (!strcmp(name, "output")) {
> -        uint32_t header;
> -        int ofs, n_bits;
> -
> -        if (nxm_parse_field_bits(value, &header, &ofs, &n_bits)[0] != '\0') {
> +        if (mf_parse_subfield(&spec->src, value)[0] != '\0') {
>             ovs_fatal(0, "%s: syntax error after NXM field name `%s'",
>                       orig, name);
>         }
>
> -        spec->n_bits = n_bits;
> +        spec->n_bits = spec->src.n_bits;
>         spec->src_type = NX_LEARN_SRC_FIELD;
> -        spec->src = mf_from_nxm_header(header);
> -        spec->src_ofs = ofs;
>         spec->dst_type = NX_LEARN_DST_OUTPUT;
> -        spec->dst = NULL;
> -        spec->dst_ofs = 0;
>     } else {
>         ovs_fatal(0, "%s: unknown keyword %s", orig, name);
>     }
> @@ -452,29 +429,29 @@ learn_parse(struct ofpbuf *b, char *arg, const struct flow *flow)
>
>             /* Check prerequisites. */
>             if (spec.src_type == NX_LEARN_SRC_FIELD
> -                && !mf_are_prereqs_ok(spec.src, flow)) {
> +                && !mf_are_prereqs_ok(spec.src.field, flow)) {
>                 ovs_fatal(0, "%s: cannot specify source field %s because "
>                           "prerequisites are not satisfied",
> -                          orig, spec.src->name);
> +                          orig, spec.src.field->name);
>             }
>             if ((spec.dst_type == NX_LEARN_DST_MATCH
>                  || spec.dst_type == NX_LEARN_DST_LOAD)
> -                && !mf_are_prereqs_ok(spec.dst, &rule.flow)) {
> +                && !mf_are_prereqs_ok(spec.dst.field, &rule.flow)) {
>                 ovs_fatal(0, "%s: cannot specify destination field %s because "
>                           "prerequisites are not satisfied",
> -                          orig, spec.dst->name);
> +                          orig, spec.dst.field->name);
>             }
>
>             /* Update 'rule' to allow for satisfying destination
>              * prerequisites. */
>             if (spec.src_type == NX_LEARN_SRC_IMMEDIATE
>                 && spec.dst_type == NX_LEARN_DST_MATCH
> -                && spec.dst_ofs == 0
> -                && spec.n_bits == spec.dst->n_bytes * 8) {
> +                && spec.dst.ofs == 0
> +                && spec.n_bits == spec.dst.field->n_bytes * 8) {
>                 union mf_value imm;
>
> -                memcpy(&imm, spec.src_imm, spec.dst->n_bytes);
> -                mf_set_value(spec.dst, &imm, &rule);
> +                memcpy(&imm, spec.src_imm, spec.dst.field->n_bytes);
> +                mf_set_value(spec.dst.field, &imm, &rule);
>             }
>
>             /* Output the flow_mod_spec. */
> @@ -486,13 +463,13 @@ learn_parse(struct ofpbuf *b, char *arg, const struct flow *flow)
>                 }
>                 ofpbuf_put(b, spec.src_imm, n_bytes);
>             } else {
> -                put_u32(b, spec.src->nxm_header);
> -                put_u16(b, spec.src_ofs);
> +                put_u32(b, spec.src.field->nxm_header);
> +                put_u16(b, spec.src.ofs);
>             }
>             if (spec.dst_type == NX_LEARN_DST_MATCH ||
>                 spec.dst_type == NX_LEARN_DST_LOAD) {
> -                put_u32(b, spec.dst->nxm_header);
> -                put_u16(b, spec.dst_ofs);
> +                put_u32(b, spec.dst.field->nxm_header);
> +                put_u16(b, spec.dst.ofs);
>             } else {
>                 assert(spec.dst_type == NX_LEARN_DST_OUTPUT);
>             }
> @@ -555,15 +532,12 @@ learn_format(const struct nx_action_learn *learn, struct ds *s)
>         int n_bits = header & NX_LEARN_N_BITS_MASK;
>
>         int src_type = header & NX_LEARN_SRC_MASK;
> -        uint32_t src_header;
> -        int src_ofs;
> +        struct mf_subfield src;
>         const uint8_t *src_value;
>         int src_value_bytes;
>
>         int dst_type = header & NX_LEARN_DST_MASK;
> -        uint32_t dst_header;
> -        int dst_ofs;
> -        const struct mf_field *dst_field;
> +        struct mf_subfield dst;
>
>         enum ofperr error;
>         int i;
> @@ -589,13 +563,13 @@ learn_format(const struct nx_action_learn *learn, struct ds *s)
>
>         /* Get the source. */
>         if (src_type == NX_LEARN_SRC_FIELD) {
> -            src_header = ntohl(get_be32(&p));
> -            src_ofs = ntohs(get_be16(&p));
> +            get_subfield(n_bits, &p, &src);
>             src_value_bytes = 0;
>             src_value = NULL;
>         } else {
> -            src_header = 0;
> -            src_ofs = 0;
> +            src.field = NULL;
> +            src.ofs = 0;
> +            src.n_bits = 0;
>             src_value_bytes = 2 * DIV_ROUND_UP(n_bits, 16);
>             src_value = p;
>             p = (const void *) ((const uint8_t *) p + src_value_bytes);
> @@ -603,41 +577,39 @@ learn_format(const struct nx_action_learn *learn, struct ds *s)
>
>         /* Get the destination. */
>         if (dst_type == NX_LEARN_DST_MATCH || dst_type == NX_LEARN_DST_LOAD) {
> -            dst_header = ntohl(get_be32(&p));
> -            dst_field = mf_from_nxm_header(dst_header);
> -            dst_ofs = ntohs(get_be16(&p));
> +            get_subfield(n_bits, &p, &dst);
>         } else {
> -            dst_header = 0;
> -            dst_field = NULL;
> -            dst_ofs = 0;
> +            dst.field = NULL;
> +            dst.ofs = 0;
> +            dst.n_bits = 0;
>         }
>
>         ds_put_char(s, ',');
>
>         switch (src_type | dst_type) {
>         case NX_LEARN_SRC_IMMEDIATE | NX_LEARN_DST_MATCH:
> -            if (dst_field && dst_ofs == 0 && n_bits == dst_field->n_bits) {
> +            if (dst.field && dst.ofs == 0 && n_bits == dst.field->n_bits) {
>                 union mf_value value;
>                 uint8_t *bytes = (uint8_t *) &value;
>
> -                if (src_value_bytes > dst_field->n_bytes) {
> +                if (src_value_bytes > dst.field->n_bytes) {
>                     /* The destination field is an odd number of bytes, which
>                      * got rounded up to a multiple of 2 to be put into the
>                      * learning action.  Skip over the leading byte, which
>                      * should be zero anyway.  Otherwise the memcpy() below
>                      * will overrun the start of 'value'. */
> -                    int diff = src_value_bytes - dst_field->n_bytes;
> +                    int diff = src_value_bytes - dst.field->n_bytes;
>                     src_value += diff;
>                     src_value_bytes -= diff;
>                 }
>
>                 memset(&value, 0, sizeof value);
> -                memcpy(&bytes[dst_field->n_bytes - src_value_bytes],
> +                memcpy(&bytes[dst.field->n_bytes - src_value_bytes],
>                        src_value, src_value_bytes);
> -                ds_put_format(s, "%s=", dst_field->name);
> -                mf_format(dst_field, &value, NULL, s);
> +                ds_put_format(s, "%s=", dst.field->name);
> +                mf_format(dst.field, &value, NULL, s);
>             } else {
> -                nxm_format_field_bits(s, dst_header, dst_ofs, n_bits);
> +                mf_format_subfield(&dst, s);
>                 ds_put_cstr(s, "=0x");
>                 for (i = 0; i < src_value_bytes; i++) {
>                     ds_put_format(s, "%02"PRIx8, src_value[i]);
> @@ -646,10 +618,10 @@ learn_format(const struct nx_action_learn *learn, struct ds *s)
>             break;
>
>         case NX_LEARN_SRC_FIELD | NX_LEARN_DST_MATCH:
> -            nxm_format_field_bits(s, dst_header, dst_ofs, n_bits);
> -            if (src_header != dst_header || src_ofs != dst_ofs) {
> +            mf_format_subfield(&dst, s);
> +            if (src.field != dst.field || src.ofs != dst.ofs) {
>                 ds_put_char(s, '=');
> -                nxm_format_field_bits(s, src_header, src_ofs, n_bits);
> +                mf_format_subfield(&src, s);
>             }
>             break;
>
> @@ -659,19 +631,19 @@ learn_format(const struct nx_action_learn *learn, struct ds *s)
>                 ds_put_format(s, "%02"PRIx8, src_value[i]);
>             }
>             ds_put_cstr(s, "->");
> -            nxm_format_field_bits(s, dst_header, dst_ofs, n_bits);
> +            mf_format_subfield(&dst, s);
>             break;
>
>         case NX_LEARN_SRC_FIELD | NX_LEARN_DST_LOAD:
>             ds_put_cstr(s, "load:");
> -            nxm_format_field_bits(s, src_header, src_ofs, n_bits);
> +            mf_format_subfield(&src, s);
>             ds_put_cstr(s, "->");
> -            nxm_format_field_bits(s, dst_header, dst_ofs, n_bits);
> +            mf_format_subfield(&dst, s);
>             break;
>
>         case NX_LEARN_SRC_FIELD | NX_LEARN_DST_OUTPUT:
>             ds_put_cstr(s, "output:");
> -            nxm_format_field_bits(s, src_header, src_ofs, n_bits);
> +            mf_format_subfield(&src, s);
>             break;
>         }
>     }
> diff --git a/lib/meta-flow.c b/lib/meta-flow.c
> index 41cbf49..e5301f3 100644
> --- a/lib/meta-flow.c
> +++ b/lib/meta-flow.c
> @@ -1,5 +1,5 @@
>  /*
> - * Copyright (c) 2011 Nicira Networks.
> + * Copyright (c) 2011, 2012 Nicira Networks.
>  *
>  * Licensed under the Apache License, Version 2.0 (the "License");
>  * you may not use this file except in compliance with the License.
> @@ -26,12 +26,16 @@
>
>  #include "classifier.h"
>  #include "dynamic-string.h"
> +#include "ofp-errors.h"
>  #include "ofp-util.h"
>  #include "packets.h"
>  #include "random.h"
>  #include "shash.h"
>  #include "socket-util.h"
>  #include "unaligned.h"
> +#include "vlog.h"
> +
> +VLOG_DEFINE_THIS_MODULE(meta_flow);
>
>  #define MF_FIELD_SIZES(MEMBER)                  \
>     sizeof ((union mf_value *)0)->MEMBER,       \
> @@ -394,6 +398,10 @@ struct nxm_field {
>
>  static struct hmap all_nxm_fields = HMAP_INITIALIZER(&all_nxm_fields);
>
> +/* Rate limit for parse errors.  These always indicate a bug in an OpenFlow
> + * controller and so there's not much point in showing a lot of them. */
> +static struct vlog_rate_limit rl = VLOG_RATE_LIMIT_INIT(1, 5);
> +
>  /* Returns the field with the given 'id'. */
>  const struct mf_field *
>  mf_from_id(enum mf_field_id id)
> @@ -1621,77 +1629,53 @@ mf_set(const struct mf_field *mf,
>     }
>  }
>
> -/* Makes a subfield starting at bit offset 'ofs' and continuing for 'n_bits' in
> - * 'rule''s field 'mf' exactly match the 'n_bits' least-significant bits of
> - * 'x'.
> - *
> - * Example: suppose that 'mf' is originally the following 2-byte field in
> - * 'rule':
> - *
> - *     value == 0xe00a == 2#1110000000001010
> - *      mask == 0xfc3f == 2#1111110000111111
> - *
> - * The call mf_set_subfield(mf, 0x55, 8, 7, rule) would have the following
> - * effect (note that 0x55 is 2#1010101):
> - *
> - *     value == 0xd50a == 2#1101010100001010
> - *      mask == 0xff3f == 2#1111111100111111
> - *
> - * The caller is responsible for ensuring that the result will be a valid
> - * wildcard pattern for 'mf'.  The caller is responsible for ensuring that
> - * 'rule' meets 'mf''s prerequisites. */
> -void
> -mf_set_subfield(const struct mf_field *mf, uint64_t x, unsigned int ofs,
> -                unsigned int n_bits, struct cls_rule *rule)
> +static enum ofperr
> +mf_check__(const struct mf_subfield *sf, const struct flow *flow,
> +           const char *type)
>  {
> -    if (ofs == 0 && mf->n_bytes * 8 == n_bits) {
> -        union mf_value value;
> -        int i;
> -
> -        for (i = mf->n_bytes - 1; i >= 0; i--) {
> -            ((uint8_t *) &value)[i] = x;
> -            x >>= 8;
> -        }
> -        mf_set_value(mf, &value, rule);
> +    if (!sf->field) {
> +        VLOG_WARN_RL(&rl, "unknown %s field", type);
> +    } else if (!sf->n_bits) {
> +        VLOG_WARN_RL(&rl, "zero bit %s field %s", type, sf->field->name);
> +    } else if (sf->ofs >= sf->field->n_bits) {
> +        VLOG_WARN_RL(&rl, "bit offset %d exceeds %d-bit width of %s field %s",
> +                     sf->ofs, sf->field->n_bits, type, sf->field->name);
> +    } else if (sf->ofs + sf->n_bits > sf->field->n_bits) {
> +        VLOG_WARN_RL(&rl, "bit offset %d and width %d exceeds %d-bit width "
> +                     "of %s field %s", sf->ofs, sf->n_bits,
> +                     sf->field->n_bits, type, sf->field->name);
> +    } else if (flow && !mf_are_prereqs_ok(sf->field, flow)) {
> +        VLOG_WARN_RL(&rl, "%s field %s lacks correct prerequisites",
> +                     type, sf->field->name);
>     } else {
> -        union mf_value value, mask;
> -        uint8_t *vp, *mp;
> -        unsigned int byte_ofs;
> -
> -        mf_get(mf, rule, &value, &mask);
> -
> -        byte_ofs = mf->n_bytes - ofs / 8;
> -        vp = &((uint8_t *) &value)[byte_ofs];
> -        mp = &((uint8_t *) &mask)[byte_ofs];
> -        if (ofs % 8) {
> -            unsigned int chunk = MIN(8 - ofs % 8, n_bits);
> -            uint8_t chunk_mask = ((1 << chunk) - 1) << (ofs % 8);
> -
> -            *--vp &= ~chunk_mask;
> -            *vp   |= chunk_mask & (x << (ofs % 8));
> -            *--mp |= chunk_mask;
> +        return 0;
> +    }
>
> -            x >>= chunk;
> -            n_bits -= chunk;
> -            ofs += chunk;
> -        }
> -        while (n_bits >= 8) {
> -            *--vp = x;
> -            *--mp = 0xff;
> -            x >>= 8;
> -            n_bits -= 8;
> -            ofs += 8;
> -        }
> -        if (n_bits) {
> -            uint8_t chunk_mask = (1 << n_bits) - 1;
> +    return OFPERR_OFPBAC_BAD_ARGUMENT;
> +}
>
> -            *--vp &= ~chunk_mask;
> -            *vp   |= chunk_mask & x;
> -            *--mp |= chunk_mask;
> -        }
> +/* Checks whether 'sf' is valid for reading a subfield out of 'flow'.  Returns
> + * 0 if so, otherwise an OpenFlow error code (e.g. as returned by
> + * ofp_mkerr()).  */
> +enum ofperr
> +mf_check_src(const struct mf_subfield *sf, const struct flow *flow)
> +{
> +    return mf_check__(sf, flow, "source");
> +}
>
> -        mf_set(mf, &value, &mask, rule);
> +/* Checks whether 'sf' is valid for writing a subfield into 'flow'.  Returns 0
> + * if so, otherwise an OpenFlow error code (e.g. as returned by
> + * ofp_mkerr()). */
> +enum ofperr
> +mf_check_dst(const struct mf_subfield *sf, const struct flow *flow)
> +{
> +    int error = mf_check__(sf, flow, "destination");
> +    if (!error && !sf->field->writable) {
> +        VLOG_WARN_RL(&rl, "destination field %s is not writable",
> +                     sf->field->name);
> +        return OFPERR_OFPBAC_BAD_ARGUMENT;
>     }
> +    return error;
>  }
>
>  /* Copies the value and wildcard bit pattern for 'mf' from 'rule' into the
> @@ -2141,3 +2125,220 @@ mf_format(const struct mf_field *mf,
>         NOT_REACHED();
>     }
>  }
> +
> +/* Makes a subfield starting at bit offset 'ofs' and continuing for 'n_bits' in
> + * 'rule''s field 'mf' exactly match the 'n_bits' least-significant bits of
> + * 'x'.
> + *
> + * Example: suppose that 'mf' is originally the following 2-byte field in
> + * 'rule':
> + *
> + *     value == 0xe00a == 2#1110000000001010
> + *      mask == 0xfc3f == 2#1111110000111111
> + *
> + * The call mf_set_subfield(mf, 0x55, 8, 7, rule) would have the following
> + * effect (note that 0x55 is 2#1010101):
> + *
> + *     value == 0xd50a == 2#1101010100001010
> + *      mask == 0xff3f == 2#1111111100111111
> + *
> + * The caller is responsible for ensuring that the result will be a valid
> + * wildcard pattern for 'mf'.  The caller is responsible for ensuring that
> + * 'rule' meets 'mf''s prerequisites. */
> +void
> +mf_set_subfield(const struct mf_subfield *sf, uint64_t x,
> +                struct cls_rule *rule)
> +{
> +    const struct mf_field *field = sf->field;
> +    unsigned int n_bits = sf->n_bits;
> +    unsigned int ofs = sf->ofs;
> +
> +    if (ofs == 0 && field->n_bytes * 8 == n_bits) {
> +        union mf_value value;
> +        int i;
> +
> +        for (i = field->n_bytes - 1; i >= 0; i--) {
> +            ((uint8_t *) &value)[i] = x;
> +            x >>= 8;
> +        }
> +        mf_set_value(field, &value, rule);
> +    } else {
> +        union mf_value value, mask;
> +        uint8_t *vp = (uint8_t *) &value;
> +        uint8_t *mp = (uint8_t *) &mask;
> +
> +        mf_get(field, rule, &value, &mask);
> +        bitwise_put(x,          vp, field->n_bytes, ofs, n_bits);
> +        bitwise_put(UINT64_MAX, mp, field->n_bytes, ofs, n_bits);
> +        mf_set(field, &value, &mask, rule);
> +    }
> +}
> +
> +/* Similar to mf_set_subfield() but modifies only a flow, not a cls_rule. */
> +void
> +mf_set_subfield_value(const struct mf_subfield *sf, uint64_t x,
> +                      struct flow *flow)
> +{
> +    const struct mf_field *field = sf->field;
> +    unsigned int n_bits = sf->n_bits;
> +    unsigned int ofs = sf->ofs;
> +    union mf_value value;
> +
> +    if (ofs == 0 && field->n_bytes * 8 == n_bits) {
> +        int i;
> +
> +        for (i = field->n_bytes - 1; i >= 0; i--) {
> +            ((uint8_t *) &value)[i] = x;
> +            x >>= 8;
> +        }
> +        mf_set_flow_value(field, &value, flow);
> +    } else {
> +        mf_get_value(field, flow, &value);
> +        bitwise_put(x, &value, field->n_bytes, ofs, n_bits);
> +        mf_set_flow_value(field, &value, flow);
> +    }
> +}
> +
> +/* Returns the value of 'sf' within 'flow'.  'sf' must be valid for reading
> + * 'flow', e.g. as checked by mf_check_src() and sf->n_bits must be 64 or
> + * less. */
> +uint64_t
> +mf_get_subfield(const struct mf_subfield *sf, const struct flow *flow)
> +{
> +    union mf_value value;
> +
> +    mf_get_value(sf->field, flow, &value);
> +    return bitwise_get(&value, sf->field->n_bytes, sf->ofs, sf->n_bits);
> +}
> +
> +/* Formats 'sf' into 's' in a format normally acceptable to
> + * mf_parse_subfield().  (It won't be acceptable if sf->field is NULL or if
> + * sf->field has no NXM name.) */
> +void
> +mf_format_subfield(const struct mf_subfield *sf, struct ds *s)
> +{
> +    if (!sf->field) {
> +        ds_put_cstr(s, "<unknown>");
> +    } else if (sf->field->nxm_name) {
> +        ds_put_cstr(s, sf->field->nxm_name);
> +    } else if (sf->field->nxm_header) {
> +        uint32_t header = sf->field->nxm_header;
> +        ds_put_format(s, "%d:%d", NXM_VENDOR(header), NXM_FIELD(header));
> +    } else {
> +        ds_put_cstr(s, sf->field->name);
> +    }
> +
> +    if (sf->ofs == 0 && sf->n_bits == sf->field->n_bits) {
> +        ds_put_cstr(s, "[]");
> +    } else if (sf->n_bits == 1) {
> +        ds_put_format(s, "[%d]", sf->ofs);
> +    } else {
> +        ds_put_format(s, "[%d..%d]", sf->ofs, sf->ofs + sf->n_bits - 1);
> +    }
> +}
> +
> +static const struct mf_field *
> +mf_parse_subfield_name(const char *name, int name_len, bool *wild)
> +{
> +    int i;
> +
> +    *wild = name_len > 2 && !memcmp(&name[name_len - 2], "_W", 2);
> +    if (*wild) {
> +        name_len -= 2;
> +    }
> +
> +    for (i = 0; i < MFF_N_IDS; i++) {
> +        const struct mf_field *mf = mf_from_id(i);
> +
> +        if (mf->nxm_name
> +            && !strncmp(mf->nxm_name, name, name_len)
> +            && mf->nxm_name[name_len] == '\0') {
> +            return mf;
> +        }
> +    }
> +
> +    return NULL;
> +}
> +
> +/* Parses a subfield from the beginning of '*s' into 'sf'.  If successful,
> + * returns NULL and advances '*s' to the first byte following the parsed
> + * string.  On failure, returns a malloc()'d error message, does not modify
> + * '*s', and does not properly initialize 'sf'.
> + *
> + * The syntax parsed from '*s' takes the form "header[start..end]" where
> + * 'header' is the name of an NXM field and 'start' and 'end' are (inclusive)
> + * bit indexes.  "..end" may be omitted to indicate a single bit.  "start..end"
> + * may both be omitted (the [] are still required) to indicate an entire
> + * field. */
> +char *
> +mf_parse_subfield__(struct mf_subfield *sf, const char **s)
> +{
> +    const char *full_s = *s;
> +    const struct mf_field *field;
> +    const char *name;
> +    int start, end;
> +    int name_len;
> +    bool wild;
> +
> +    name = *s;
> +    name_len = strcspn(*s, "[");
> +    if ((*s)[name_len] != '[') {
> +        return xasprintf("%s: missing [ looking for field name", full_s);
> +    }
> +
> +    field = mf_parse_subfield_name(name, name_len, &wild);
> +    if (!field) {
> +        return xasprintf("%s: unknown field `%.*s'", full_s, name_len, *s);
> +    }
> +
> +    *s += name_len;
> +    if (sscanf(*s, "[%d..%d]", &start, &end) == 2) {
> +        /* Nothing to do. */
> +    } else if (sscanf(*s, "[%d]", &start) == 1) {
> +        end = start;
> +    } else if (!strncmp(*s, "[]", 2)) {
> +        start = 0;
> +        end = field->n_bits - 1;
> +    } else {
> +        return xasprintf("%s: syntax error expecting [] or [<bit>] or "
> +                         "[<start>..<end>]", full_s);
> +    }
> +    *s = strchr(*s, ']') + 1;
> +
> +    if (start > end) {
> +        return xasprintf("%s: starting bit %d is after ending bit %d",
> +                         full_s, start, end);
> +    } else if (start >= field->n_bits) {
> +        return xasprintf("%s: starting bit %d is not valid because field is "
> +                         "only %d bits wide", full_s, start, field->n_bits);
> +    } else if (end >= field->n_bits){
> +        return xasprintf("%s: ending bit %d is not valid because field is "
> +                         "only %d bits wide", full_s, end, field->n_bits);
> +    }
> +
> +    sf->field = field;
> +    sf->ofs = start;
> +    sf->n_bits = end - start + 1;
> +
> +    return NULL;
> +}
> +
> +/* Parses a subfield from the beginning of 's' into 'sf'.  Returns the first
> + * byte in 's' following the parsed string.
> + *
> + * Exits with an error message if 's' has incorrect syntax.
> + *
> + * The syntax parsed from 's' takes the form "header[start..end]" where
> + * 'header' is the name of an NXM field and 'start' and 'end' are (inclusive)
> + * bit indexes.  "..end" may be omitted to indicate a single bit.  "start..end"
> + * may both be omitted (the [] are still required) to indicate an entire
> + * field.  */
> +const char *
> +mf_parse_subfield(struct mf_subfield *sf, const char *s)
> +{
> +    char *msg = mf_parse_subfield__(sf, &s);
> +    if (msg) {
> +        ovs_fatal(0, "%s", msg);
> +    }
> +    return s;
> +}
> diff --git a/lib/meta-flow.h b/lib/meta-flow.h
> index a552e3c..4917834 100644
> --- a/lib/meta-flow.h
> +++ b/lib/meta-flow.h
> @@ -1,5 +1,5 @@
>  /*
> - * Copyright (c) 2011 Nicira Networks.
> + * Copyright (c) 2011, 2012 Nicira Networks.
>  *
>  * Licensed under the Apache License, Version 2.0 (the "License");
>  * you may not use this file except in compliance with the License.
> @@ -21,6 +21,7 @@
>  #include <netinet/in.h>
>  #include <netinet/ip6.h>
>  #include "flow.h"
> +#include "ofp-errors.h"
>  #include "packets.h"
>
>  struct cls_rule;
> @@ -198,6 +199,13 @@ union mf_value {
>     struct in6_addr ipv6;
>  };
>
> +/* Part of a field. */
> +struct mf_subfield {
> +    const struct mf_field *field;
> +    unsigned int ofs;           /* Bit offset. */
> +    unsigned int n_bits;        /* Number of bits. */
> +};
> +
>  /* Finding mf_fields. */
>  const struct mf_field *mf_from_id(enum mf_field_id);
>  const struct mf_field *mf_from_name(const char *name);
> @@ -230,13 +238,25 @@ void mf_get(const struct mf_field *, const struct cls_rule *,
>  void mf_set(const struct mf_field *,
>             const union mf_value *value, const union mf_value *mask,
>             struct cls_rule *);
> -void mf_set_subfield(const struct mf_field *, uint64_t value, unsigned int ofs,
> -                     unsigned int n_bits, struct cls_rule *);
>
>  void mf_set_wild(const struct mf_field *, struct cls_rule *);
>
>  void mf_random_value(const struct mf_field *, union mf_value *value);
>
> +/* Subfields. */
> +void mf_set_subfield(const struct mf_subfield *, uint64_t value,
> +                     struct cls_rule *);
> +void mf_set_subfield_value(const struct mf_subfield *, uint64_t value,
> +                           struct flow *);
> +uint64_t mf_get_subfield(const struct mf_subfield *, const struct flow *);
> +
> +void mf_format_subfield(const struct mf_subfield *, struct ds *);
> +char *mf_parse_subfield__(struct mf_subfield *sf, const char **s);
> +const char *mf_parse_subfield(struct mf_subfield *, const char *);
> +
> +enum ofperr mf_check_src(const struct mf_subfield *, const struct flow *);
> +enum ofperr mf_check_dst(const struct mf_subfield *, const struct flow *);
> +
>  /* Parsing and formatting. */
>  char *mf_parse(const struct mf_field *, const char *,
>                union mf_value *value, union mf_value *mask);
> diff --git a/lib/multipath.c b/lib/multipath.c
> index 5175803..46976fb 100644
> --- a/lib/multipath.c
> +++ b/lib/multipath.c
> @@ -1,5 +1,5 @@
>  /*
> - * Copyright (c) 2010, 2011 Nicira Networks.
> + * Copyright (c) 2010, 2011, 2012 Nicira Networks.
>  *
>  * Licensed under the Apache License, Version 2.0 (the "License");
>  * you may not use this file except in compliance with the License.
> @@ -22,6 +22,7 @@
>  #include <sys/types.h>
>  #include <netinet/in.h>
>  #include "dynamic-string.h"
> +#include "meta-flow.h"
>  #include "nx-match.h"
>  #include "ofp-errors.h"
>  #include "ofp-util.h"
> @@ -39,11 +40,11 @@ multipath_check(const struct nx_action_multipath *mp, const struct flow *flow)
>  {
>     uint32_t n_links = ntohs(mp->max_link) + 1;
>     size_t min_n_bits = log_2_ceil(n_links);
> -    int ofs = nxm_decode_ofs(mp->ofs_nbits);
> -    int n_bits = nxm_decode_n_bits(mp->ofs_nbits);
> +    struct mf_subfield dst;
>     enum ofperr error;
>
> -    error = nxm_dst_check(mp->dst, ofs, n_bits, flow);
> +    nxm_decode(&dst, mp->dst, mp->ofs_nbits);
> +    error = mf_check_dst(&dst, flow);
>     if (error) {
>         return error;
>     }
> @@ -56,7 +57,7 @@ multipath_check(const struct nx_action_multipath *mp, const struct flow *flow)
>                && mp->algorithm != htons(NX_MP_ALG_ITER_HASH)) {
>         VLOG_WARN_RL(&rl, "unsupported algorithm %"PRIu16,
>                      ntohs(mp->algorithm));
> -    } else if (n_bits < min_n_bits) {
> +    } else if (dst.n_bits < min_n_bits) {
>         VLOG_WARN_RL(&rl, "multipath action requires at least %zu bits for "
>                      "%"PRIu32" links", min_n_bits, n_links);
>     } else {
> @@ -80,8 +81,10 @@ multipath_execute(const struct nx_action_multipath *mp, struct flow *flow)
>     uint16_t link = multipath_algorithm(hash, ntohs(mp->algorithm),
>                                         ntohs(mp->max_link) + 1,
>                                         ntohl(mp->arg));
> +    struct mf_subfield dst;
>
> -    nxm_reg_load(mp->dst, mp->ofs_nbits, link, flow);
> +    nxm_decode(&dst, mp->dst, mp->ofs_nbits);
> +    mf_set_subfield_value(&dst, link, flow);
>  }
>
>  static uint16_t
> @@ -166,9 +169,8 @@ multipath_parse(struct nx_action_multipath *mp, const char *s_)
>  {
>     char *s = xstrdup(s_);
>     char *save_ptr = NULL;
> -    char *fields, *basis, *algorithm, *n_links_str, *arg, *dst;
> -    uint32_t header;
> -    int ofs, n_bits;
> +    char *fields, *basis, *algorithm, *n_links_str, *arg, *dst_s;
> +    struct mf_subfield dst;
>     int n_links;
>
>     fields = strtok_r(s, ", ", &save_ptr);
> @@ -176,8 +178,8 @@ multipath_parse(struct nx_action_multipath *mp, const char *s_)
>     algorithm = strtok_r(NULL, ", ", &save_ptr);
>     n_links_str = strtok_r(NULL, ", ", &save_ptr);
>     arg = strtok_r(NULL, ", ", &save_ptr);
> -    dst = strtok_r(NULL, ", ", &save_ptr);
> -    if (!dst) {
> +    dst_s = strtok_r(NULL, ", ", &save_ptr);
> +    if (!dst_s) {
>         ovs_fatal(0, "%s: not enough arguments to multipath action", s_);
>     }
>
> @@ -209,14 +211,14 @@ multipath_parse(struct nx_action_multipath *mp, const char *s_)
>     mp->max_link = htons(n_links - 1);
>     mp->arg = htonl(atoi(arg));
>
> -    nxm_parse_field_bits(dst, &header, &ofs, &n_bits);
> -    if (n_bits < 16 && n_links > (1u << n_bits)) {
> +    mf_parse_subfield(&dst, dst_s);
> +    if (dst.n_bits < 16 && n_links > (1u << dst.n_bits)) {
>         ovs_fatal(0, "%s: %d-bit destination field has %u possible values, "
>                   "less than specified n_links %d",
> -                  s_, n_bits, 1u << n_bits, n_links);
> +                  s_, dst.n_bits, 1u << dst.n_bits, n_links);
>     }
> -    mp->ofs_nbits = nxm_encode_ofs_nbits(ofs, n_bits);
> -    mp->dst = htonl(header);
> +    mp->ofs_nbits = nxm_encode_ofs_nbits(dst.ofs, dst.n_bits);
> +    mp->dst = htonl(dst.field->nxm_header);
>
>     free(s);
>  }
> @@ -229,6 +231,8 @@ multipath_format(const struct nx_action_multipath *mp, struct ds *s)
>     uint16_t mp_fields    = ntohs(mp->fields);
>     uint16_t mp_algorithm = ntohs(mp->algorithm);
>
> +    struct mf_subfield dst;
> +
>     fields = flow_hash_fields_to_str(mp_fields);
>
>     switch ((enum nx_mp_algorithm) mp_algorithm) {
> @@ -251,7 +255,7 @@ multipath_format(const struct nx_action_multipath *mp, struct ds *s)
>     ds_put_format(s, "multipath(%s,%"PRIu16",%s,%d,%"PRIu16",",
>                   fields, ntohs(mp->basis), algorithm, ntohs(mp->max_link) + 1,
>                   ntohl(mp->arg));
> -    nxm_format_field_bits(s, ntohl(mp->dst), nxm_decode_ofs(mp->ofs_nbits),
> -                          nxm_decode_n_bits(mp->ofs_nbits));
> +    nxm_decode(&dst, mp->dst, mp->ofs_nbits);
> +    mf_format_subfield(&dst, s);
>     ds_put_char(s, ')');
>  }
> diff --git a/lib/nx-match.c b/lib/nx-match.c
> index ef166c7..9d3b2be 100644
> --- a/lib/nx-match.c
> +++ b/lib/nx-match.c
> @@ -794,97 +794,40 @@ nx_match_from_string(const char *s, struct ofpbuf *b)
>     return match_len;
>  }
>
> -const char *
> -nxm_parse_field_bits(const char *s, uint32_t *headerp, int *ofsp, int *n_bitsp)
> -{
> -    const char *full_s = s;
> -    const char *name;
> -    uint32_t header;
> -    int start, end;
> -    int name_len;
> -    int width;
> -
> -    name = s;
> -    name_len = strcspn(s, "[");
> -    if (s[name_len] != '[') {
> -        ovs_fatal(0, "%s: missing [ looking for field name", full_s);
> -    }
> -
> -    header = parse_nxm_field_name(name, name_len);
> -    if (!header) {
> -        ovs_fatal(0, "%s: unknown field `%.*s'", full_s, name_len, s);
> -    }
> -    width = nxm_field_bits(header);
> -
> -    s += name_len;
> -    if (sscanf(s, "[%d..%d]", &start, &end) == 2) {
> -        /* Nothing to do. */
> -    } else if (sscanf(s, "[%d]", &start) == 1) {
> -        end = start;
> -    } else if (!strncmp(s, "[]", 2)) {
> -        start = 0;
> -        end = width - 1;
> -    } else {
> -        ovs_fatal(0, "%s: syntax error expecting [] or [<bit>] or "
> -                  "[<start>..<end>]", full_s);
> -    }
> -    s = strchr(s, ']') + 1;
> -
> -    if (start > end) {
> -        ovs_fatal(0, "%s: starting bit %d is after ending bit %d",
> -                  full_s, start, end);
> -    } else if (start >= width) {
> -        ovs_fatal(0, "%s: starting bit %d is not valid because field is only "
> -                  "%d bits wide", full_s, start, width);
> -    } else if (end >= width){
> -        ovs_fatal(0, "%s: ending bit %d is not valid because field is only "
> -                  "%d bits wide", full_s, end, width);
> -    }
> -
> -    *headerp = header;
> -    *ofsp = start;
> -    *n_bitsp = end - start + 1;
> -
> -    return s;
> -}
> -
>  void
>  nxm_parse_reg_move(struct nx_action_reg_move *move, const char *s)
>  {
>     const char *full_s = s;
> -    uint32_t src, dst;
> -    int src_ofs, dst_ofs;
> -    int src_n_bits, dst_n_bits;
> +    struct mf_subfield src, dst;
>
> -    s = nxm_parse_field_bits(s, &src, &src_ofs, &src_n_bits);
> +    s = mf_parse_subfield(&src, s);
>     if (strncmp(s, "->", 2)) {
>         ovs_fatal(0, "%s: missing `->' following source", full_s);
>     }
>     s += 2;
> -    s = nxm_parse_field_bits(s, &dst, &dst_ofs, &dst_n_bits);
> +    s = mf_parse_subfield(&dst, s);
>     if (*s != '\0') {
>         ovs_fatal(0, "%s: trailing garbage following destination", full_s);
>     }
>
> -    if (src_n_bits != dst_n_bits) {
> +    if (src.n_bits != dst.n_bits) {
>         ovs_fatal(0, "%s: source field is %d bits wide but destination is "
> -                  "%d bits wide", full_s, src_n_bits, dst_n_bits);
> +                  "%d bits wide", full_s, src.n_bits, dst.n_bits);
>     }
>
>     ofputil_init_NXAST_REG_MOVE(move);
> -    move->n_bits = htons(src_n_bits);
> -    move->src_ofs = htons(src_ofs);
> -    move->dst_ofs = htons(dst_ofs);
> -    move->src = htonl(src);
> -    move->dst = htonl(dst);
> +    move->n_bits = htons(src.n_bits);
> +    move->src_ofs = htons(src.ofs);
> +    move->dst_ofs = htons(dst.ofs);
> +    move->src = htonl(src.field->nxm_header);
> +    move->dst = htonl(dst.field->nxm_header);
>  }
>
>  void
>  nxm_parse_reg_load(struct nx_action_reg_load *load, const char *s)
>  {
>     const char *full_s = s;
> -    uint32_t dst;
> -    int ofs, n_bits;
> +    struct mf_subfield dst;
>     uint64_t value;
>
>     value = strtoull(s, (char **) &s, 0);
> @@ -892,151 +835,85 @@ nxm_parse_reg_load(struct nx_action_reg_load *load, const char *s)
>         ovs_fatal(0, "%s: missing `->' following value", full_s);
>     }
>     s += 2;
> -    s = nxm_parse_field_bits(s, &dst, &ofs, &n_bits);
> +    s = mf_parse_subfield(&dst, s);
>     if (*s != '\0') {
>         ovs_fatal(0, "%s: trailing garbage following destination", full_s);
>     }
>
> -    if (n_bits < 64 && (value >> n_bits) != 0) {
> -        ovs_fatal(0, "%s: value %"PRIu64" does not fit into %d bits",
> -                  full_s, value, n_bits);
> +    if (dst.n_bits < 64 && (value >> dst.n_bits) != 0) {
> +        ovs_fatal(0, "%s: value %"PRIu64" does not fit into %u bits",
> +                  full_s, value, dst.n_bits);
>     }
>
>     ofputil_init_NXAST_REG_LOAD(load);
> -    load->ofs_nbits = nxm_encode_ofs_nbits(ofs, n_bits);
> -    load->dst = htonl(dst);
> +    load->ofs_nbits = nxm_encode_ofs_nbits(dst.ofs, dst.n_bits);
> +    load->dst = htonl(dst.field->nxm_header);
>     load->value = htonll(value);
>  }
>
>  /* nxm_format_reg_move(), nxm_format_reg_load(). */
>
>  void
> -nxm_format_field_bits(struct ds *s, uint32_t header, int ofs, int n_bits)
> -{
> -    format_nxm_field_name(s, header);
> -    if (ofs == 0 && n_bits == nxm_field_bits(header)) {
> -        ds_put_cstr(s, "[]");
> -    } else if (n_bits == 1) {
> -        ds_put_format(s, "[%d]", ofs);
> -    } else {
> -        ds_put_format(s, "[%d..%d]", ofs, ofs + n_bits - 1);
> -    }
> -}
> -
> -void
>  nxm_format_reg_move(const struct nx_action_reg_move *move, struct ds *s)
>  {
> -    int n_bits = ntohs(move->n_bits);
> -    int src_ofs = ntohs(move->src_ofs);
> -    int dst_ofs = ntohs(move->dst_ofs);
> -    uint32_t src = ntohl(move->src);
> -    uint32_t dst = ntohl(move->dst);
> +    struct mf_subfield src, dst;
> +
> +    nxm_decode_discrete(&src, move->src, move->src_ofs, move->n_bits);
> +    nxm_decode_discrete(&dst, move->dst, move->dst_ofs, move->n_bits);
>
>     ds_put_format(s, "move:");
> -    nxm_format_field_bits(s, src, src_ofs, n_bits);
> +    mf_format_subfield(&src, s);
>     ds_put_cstr(s, "->");
> -    nxm_format_field_bits(s, dst, dst_ofs, n_bits);
> +    mf_format_subfield(&dst, s);
>  }
>
>  void
>  nxm_format_reg_load(const struct nx_action_reg_load *load, struct ds *s)
>  {
> -    int ofs = nxm_decode_ofs(load->ofs_nbits);
> -    int n_bits = nxm_decode_n_bits(load->ofs_nbits);
> -    uint32_t dst = ntohl(load->dst);
> -    uint64_t value = ntohll(load->value);
> +    struct mf_subfield dst;
>
> -    ds_put_format(s, "load:%#"PRIx64"->", value);
> -    nxm_format_field_bits(s, dst, ofs, n_bits);
> +    ds_put_format(s, "load:%#"PRIx64"->", ntohll(load->value));
> +
> +    nxm_decode(&dst, load->dst, load->ofs_nbits);
> +    mf_format_subfield(&dst, s);
>  }
>
>  /* nxm_check_reg_move(), nxm_check_reg_load(). */
>
> -static bool
> -field_ok(const struct mf_field *mf, const struct flow *flow, int size)
> -{
> -    return (mf
> -            && mf_are_prereqs_ok(mf, flow)
> -            && size <= nxm_field_bits(mf->nxm_header));
> -}
> -
> -int
> +enum ofperr
>  nxm_check_reg_move(const struct nx_action_reg_move *action,
>                    const struct flow *flow)
>  {
> -    int src_ofs, dst_ofs, n_bits;
> +    struct mf_subfield src;
> +    struct mf_subfield dst;
>     int error;
>
> -    n_bits = ntohs(action->n_bits);
> -    src_ofs = ntohs(action->src_ofs);
> -    dst_ofs = ntohs(action->dst_ofs);
> -
> -    error = nxm_src_check(action->src, src_ofs, n_bits, flow);
> +    nxm_decode_discrete(&src, action->src, action->src_ofs, action->n_bits);
> +    error = mf_check_src(&src, flow);
>     if (error) {
>         return error;
>     }
>
> -    return nxm_dst_check(action->dst, dst_ofs, n_bits, flow);
> -}
> -
> -/* Given a flow, checks that the source field represented by 'src_header'
> - * in the range ['ofs', 'ofs' + 'n_bits') is valid. */
> -enum ofperr
> -nxm_src_check(ovs_be32 src_header_, unsigned int ofs, unsigned int n_bits,
> -              const struct flow *flow)
> -{
> -    uint32_t src_header = ntohl(src_header_);
> -    const struct mf_field *src = mf_from_nxm_header(src_header);
> -
> -    if (!n_bits) {
> -        VLOG_WARN_RL(&rl, "zero bit source field");
> -    } else if (NXM_HASMASK(src_header) || !field_ok(src, flow, ofs + n_bits)) {
> -        VLOG_WARN_RL(&rl, "invalid source field");
> -    } else {
> -        return 0;
> -    }
> -
> -    return OFPERR_OFPBAC_BAD_ARGUMENT;
> -}
> -
> -/* Given a flow, checks that the destination field represented by 'dst_header'
> - * in the range ['ofs', 'ofs' + 'n_bits') is valid. */
> -enum ofperr
> -nxm_dst_check(ovs_be32 dst_header_, unsigned int ofs, unsigned int n_bits,
> -              const struct flow *flow)
> -{
> -    uint32_t dst_header = ntohl(dst_header_);
> -    const struct mf_field *dst = mf_from_nxm_header(dst_header);
> -
> -    if (!n_bits) {
> -        VLOG_WARN_RL(&rl, "zero bit destination field");
> -    } else if (NXM_HASMASK(dst_header) || !field_ok(dst, flow, ofs + n_bits)) {
> -        VLOG_WARN_RL(&rl, "invalid destination field");
> -    } else if (!dst->writable) {
> -        VLOG_WARN_RL(&rl, "destination field is not writable");
> -    } else {
> -        return 0;
> -    }
> -
> -    return OFPERR_OFPBAC_BAD_ARGUMENT;
> +    nxm_decode_discrete(&dst, action->dst, action->dst_ofs, action->n_bits);
> +    return mf_check_dst(&dst, flow);
>  }
>
>  enum ofperr
>  nxm_check_reg_load(const struct nx_action_reg_load *action,
>                    const struct flow *flow)
>  {
> -    unsigned int ofs = nxm_decode_ofs(action->ofs_nbits);
> -    unsigned int n_bits = nxm_decode_n_bits(action->ofs_nbits);
> +    struct mf_subfield dst;
>     enum ofperr error;
>
> -    error = nxm_dst_check(action->dst, ofs, n_bits, flow);
> +    nxm_decode(&dst, action->dst, action->ofs_nbits);
> +    error = mf_check_dst(&dst, flow);
>     if (error) {
>         return error;
>     }
>
>     /* Reject 'action' if a bit numbered 'n_bits' or higher is set to 1 in
>      * action->value. */
> -    if (n_bits < 64 && ntohll(action->value) >> n_bits) {
> +    if (dst.n_bits < 64 && ntohll(action->value) >> dst.n_bits) {
>         return OFPERR_OFPBAC_BAD_ARGUMENT;
>     }
>
> @@ -1045,64 +922,72 @@ nxm_check_reg_load(const struct nx_action_reg_load *action,
>
>  /* nxm_execute_reg_move(), nxm_execute_reg_load(). */
>
> -/* Returns the value of the NXM field corresponding to 'header' at 'ofs_nbits'
> - * in 'flow'. */
> -uint64_t
> -nxm_read_field_bits(ovs_be32 header, ovs_be16 ofs_nbits,
> -                    const struct flow *flow)
> -{
> -    const struct mf_field *field = mf_from_nxm_header(ntohl(header));
> -    union mf_value value;
> -    union mf_value bits;
> -
> -    mf_get_value(field, flow, &value);
> -    bits.be64 = htonll(0);
> -    bitwise_copy(&value, field->n_bytes, nxm_decode_ofs(ofs_nbits),
> -                 &bits, sizeof bits.be64, 0,
> -                 nxm_decode_n_bits(ofs_nbits));
> -    return ntohll(bits.be64);
> -}
> -
>  void
>  nxm_execute_reg_move(const struct nx_action_reg_move *action,
>                      struct flow *flow)
>  {
> -    const struct mf_field *src = mf_from_nxm_header(ntohl(action->src));
> -    const struct mf_field *dst = mf_from_nxm_header(ntohl(action->dst));
> +    struct mf_subfield src, dst;
>     union mf_value src_value;
>     union mf_value dst_value;
>
> -    mf_get_value(dst, flow, &dst_value);
> -    mf_get_value(src, flow, &src_value);
> -    bitwise_copy(&src_value, src->n_bytes, ntohs(action->src_ofs),
> -                 &dst_value, dst->n_bytes, ntohs(action->dst_ofs),
> -                 ntohs(action->n_bits));
> -    mf_set_flow_value(dst, &dst_value, flow);
> +    nxm_decode_discrete(&src, action->src, action->src_ofs, action->n_bits);
> +    nxm_decode_discrete(&dst, action->dst, action->dst_ofs, action->n_bits);
> +
> +    mf_get_value(dst.field, flow, &dst_value);
> +    mf_get_value(src.field, flow, &src_value);
> +    bitwise_copy(&src_value, src.field->n_bytes, src.ofs,
> +                 &dst_value, dst.field->n_bytes, dst.ofs,
> +                 src.n_bits);
> +    mf_set_flow_value(dst.field, &dst_value, flow);
>  }
>
>  void
>  nxm_execute_reg_load(const struct nx_action_reg_load *action,
>                      struct flow *flow)
>  {
> -    nxm_reg_load(action->dst, action->ofs_nbits, ntohll(action->value), flow);
> +    struct mf_subfield dst;
> +
> +    nxm_decode(&dst, action->dst, action->ofs_nbits);
> +    mf_set_subfield_value(&dst, ntohll(action->value), flow);
>  }
>
> -/* Calculates ofs and n_bits from the given 'ofs_nbits' parameter, and copies
> - * 'src_data'[0:n_bits] to 'dst_header'[ofs:ofs+n_bits] in the given 'flow'. */
> +/* Initializes 'sf->field' with the field corresponding to the given NXM
> + * 'header' and 'sf->ofs' and 'sf->n_bits' decoded from 'ofs_nbits' with
> + * nxm_decode_ofs() and nxm_decode_n_bits(), respectively.
> + *
> + * Afterward, 'sf' might be invalid in a few different ways:
> + *
> + *   - 'sf->field' will be NULL if 'header' is unknown.
> + *
> + *   - 'sf->ofs' and 'sf->n_bits' might exceed the width of sf->field.
> + *
> + * The caller should call mf_check_src() or mf_check_dst() to check for these
> + * problems. */
>  void
> -nxm_reg_load(ovs_be32 dst_header, ovs_be16 ofs_nbits, uint64_t src_data,
> -             struct flow *flow)
> +nxm_decode(struct mf_subfield *sf, ovs_be32 header, ovs_be16 ofs_nbits)
>  {
> -    const struct mf_field *dst = mf_from_nxm_header(ntohl(dst_header));
> -    int n_bits = nxm_decode_n_bits(ofs_nbits);
> -    int dst_ofs = nxm_decode_ofs(ofs_nbits);
> -    union mf_value dst_value;
> -    union mf_value src_value;
> +    sf->field = mf_from_nxm_header(ntohl(header));
> +    sf->ofs = nxm_decode_ofs(ofs_nbits);
> +    sf->n_bits = nxm_decode_n_bits(ofs_nbits);
> +}
>
> -    mf_get_value(dst, flow, &dst_value);
> -    src_value.be64 = htonll(src_data);
> -    bitwise_copy(&src_value, sizeof src_value.be64, 0,
> -                 &dst_value, dst->n_bytes, dst_ofs,
> -                 n_bits);
> -    mf_set_flow_value(dst, &dst_value, flow);
> +/* Initializes 'sf->field' with the field corresponding to the given NXM
> + * 'header' and 'sf->ofs' and 'sf->n_bits' from 'ofs' and 'n_bits',
> + * respectively.
> + *
> + * Afterward, 'sf' might be invalid in a few different ways:
> + *
> + *   - 'sf->field' will be NULL if 'header' is unknown.
> + *
> + *   - 'sf->ofs' and 'sf->n_bits' might exceed the width of sf->field.
> + *
> + * The caller should call mf_check_src() or mf_check_dst() to check for these
> + * problems. */
> +void
> +nxm_decode_discrete(struct mf_subfield *sf, ovs_be32 header,
> +                    ovs_be16 ofs, ovs_be16 n_bits)
> +{
> +    sf->field = mf_from_nxm_header(ntohl(header));
> +    sf->ofs = ntohs(ofs);
> +    sf->n_bits = ntohs(n_bits);
>  }
> diff --git a/lib/nx-match.h b/lib/nx-match.h
> index c189287..3406e04 100644
> --- a/lib/nx-match.h
> +++ b/lib/nx-match.h
> @@ -26,6 +26,7 @@
>  struct cls_rule;
>  struct ds;
>  struct flow;
> +struct mf_subfield;
>  struct ofpbuf;
>  struct nx_action_reg_load;
>  struct nx_action_reg_move;
> @@ -47,37 +48,24 @@ int nx_put_match(struct ofpbuf *, const struct cls_rule *,
>  char *nx_match_to_string(const uint8_t *, unsigned int match_len);
>  int nx_match_from_string(const char *, struct ofpbuf *);
>
> -uint64_t nxm_read_field_bits(ovs_be32 header, ovs_be16 ofs_nbits,
> -                             const struct flow *);
> -
>  void nxm_parse_reg_move(struct nx_action_reg_move *, const char *);
>  void nxm_parse_reg_load(struct nx_action_reg_load *, const char *);
>
>  void nxm_format_reg_move(const struct nx_action_reg_move *, struct ds *);
>  void nxm_format_reg_load(const struct nx_action_reg_load *, struct ds *);
>
> -int nxm_check_reg_move(const struct nx_action_reg_move *, const struct flow *);
> +enum ofperr nxm_check_reg_move(const struct nx_action_reg_move *,
> +                               const struct flow *);
>  enum ofperr nxm_check_reg_load(const struct nx_action_reg_load *,
>                                const struct flow *);
> -enum ofperr nxm_src_check(ovs_be32 src, unsigned int ofs, unsigned int n_bits,
> -                          const struct flow *);
> -enum ofperr nxm_dst_check(ovs_be32 dst, unsigned int ofs, unsigned int n_bits,
> -                          const struct flow *);
>
>  void nxm_execute_reg_move(const struct nx_action_reg_move *, struct flow *);
>  void nxm_execute_reg_load(const struct nx_action_reg_load *, struct flow *);
> -void nxm_reg_load(ovs_be32 dst, ovs_be16 ofs_nbits, uint64_t src_data,
> -                  struct flow *);
>
>  int nxm_field_bytes(uint32_t header);
>  int nxm_field_bits(uint32_t header);
>
> -const char *nxm_parse_field_bits(const char *s,
> -                                 uint32_t *headerp, int *ofsp, int *n_bitsp);
> -void nxm_format_field_bits(struct ds *, uint32_t header, int ofs, int n_bits);
> -
> -/* Dealing with the 'ofs_nbits' members of struct nx_action_reg_load and struct
> - * nx_action_multipath. */
> +/* Dealing with the 'ofs_nbits' members in several Nicira extensions. */
>
>  static inline ovs_be16
>  nxm_encode_ofs_nbits(int ofs, int n_bits)
> @@ -96,6 +84,10 @@ nxm_decode_n_bits(ovs_be16 ofs_nbits)
>  {
>     return (ntohs(ofs_nbits) & 0x3f) + 1;
>  }
> +
> +void nxm_decode(struct mf_subfield *, ovs_be32 header, ovs_be16 ofs_nbits);
> +void nxm_decode_discrete(struct mf_subfield *, ovs_be32 header,
> +                         ovs_be16 ofs, ovs_be16 n_bits);
>
>  /* Upper bound on the length of an nx_match.  The longest nx_match (an
>  * IPV6 neighbor discovery message using 5 registers) would be:
> diff --git a/lib/ofp-parse.c b/lib/ofp-parse.c
> index 38c3dab..39226a3 100644
> --- a/lib/ofp-parse.c
> +++ b/lib/ofp-parse.c
> @@ -151,14 +151,13 @@ parse_output(struct ofpbuf *b, char *arg)
>  {
>     if (strchr(arg, '[')) {
>         struct nx_action_output_reg *naor;
> -        int ofs, n_bits;
> -        uint32_t src;
> +        struct mf_subfield src;
>
> -        nxm_parse_field_bits(arg, &src, &ofs, &n_bits);
> +        mf_parse_subfield(&src, arg);
>
>         naor = ofputil_put_NXAST_OUTPUT_REG(b);
> -        naor->ofs_nbits = nxm_encode_ofs_nbits(ofs, n_bits);
> -        naor->src = htonl(src);
> +        naor->ofs_nbits = nxm_encode_ofs_nbits(src.ofs, src.n_bits);
> +        naor->src = htonl(src.field->nxm_header);
>         naor->max_len = htons(UINT16_MAX);
>     } else {
>         put_output_action(b, str_to_u32(arg));
> diff --git a/lib/ofp-print.c b/lib/ofp-print.c
> index aff12b6..d7371cf 100644
> --- a/lib/ofp-print.c
> +++ b/lib/ofp-print.c
> @@ -33,6 +33,7 @@
>  #include "flow.h"
>  #include "learn.h"
>  #include "multipath.h"
> +#include "meta-flow.h"
>  #include "nx-match.h"
>  #include "ofp-errors.h"
>  #include "ofp-util.h"
> @@ -178,6 +179,7 @@ ofp_print_action(struct ds *s, const union ofp_action *a,
>     const struct nx_action_multipath *nam;
>     const struct nx_action_autopath *naa;
>     const struct nx_action_output_reg *naor;
> +    struct mf_subfield subfield;
>     uint16_t port;
>
>     switch (code) {
> @@ -310,9 +312,8 @@ ofp_print_action(struct ds *s, const union ofp_action *a,
>     case OFPUTIL_NXAST_AUTOPATH:
>         naa = (const struct nx_action_autopath *)a;
>         ds_put_format(s, "autopath(%u,", ntohl(naa->id));
> -        nxm_format_field_bits(s, ntohl(naa->dst),
> -                              nxm_decode_ofs(naa->ofs_nbits),
> -                              nxm_decode_n_bits(naa->ofs_nbits));
> +        nxm_decode(&subfield, naa->dst, naa->ofs_nbits);
> +        mf_format_subfield(&subfield, s);
>         ds_put_char(s, ')');
>         break;
>
> @@ -324,9 +325,8 @@ ofp_print_action(struct ds *s, const union ofp_action *a,
>     case OFPUTIL_NXAST_OUTPUT_REG:
>         naor = (const struct nx_action_output_reg *) a;
>         ds_put_cstr(s, "output:");
> -        nxm_format_field_bits(s, ntohl(naor->src),
> -                              nxm_decode_ofs(naor->ofs_nbits),
> -                              nxm_decode_n_bits(naor->ofs_nbits));
> +        nxm_decode(&subfield, naor->src, naor->ofs_nbits);
> +        mf_format_subfield(&subfield, s);
>         break;
>
>     case OFPUTIL_NXAST_LEARN:
> diff --git a/lib/ofp-util.c b/lib/ofp-util.c
> index b20d3fb..dc5c30c 100644
> --- a/lib/ofp-util.c
> +++ b/lib/ofp-util.c
> @@ -29,6 +29,7 @@
>  #include "dynamic-string.h"
>  #include "learn.h"
>  #include "multipath.h"
> +#include "meta-flow.h"
>  #include "nx-match.h"
>  #include "ofp-errors.h"
>  #include "ofp-util.h"
> @@ -2320,6 +2321,7 @@ static enum ofperr
>  check_output_reg(const struct nx_action_output_reg *naor,
>                  const struct flow *flow)
>  {
> +    struct mf_subfield src;
>     size_t i;
>
>     for (i = 0; i < sizeof naor->zero; i++) {
> @@ -2328,8 +2330,8 @@ check_output_reg(const struct nx_action_output_reg *naor,
>         }
>     }
>
> -    return nxm_src_check(naor->src, nxm_decode_ofs(naor->ofs_nbits),
> -                         nxm_decode_n_bits(naor->ofs_nbits), flow);
> +    nxm_decode(&src, naor->src, naor->ofs_nbits);
> +    return mf_check_src(&src, flow);
>  }
>
>  enum ofperr
> diff --git a/ofproto/ofproto-dpif.c b/ofproto/ofproto-dpif.c
> index 6ecf71b..633fa95 100644
> --- a/ofproto/ofproto-dpif.c
> +++ b/ofproto/ofproto-dpif.c
> @@ -34,6 +34,7 @@
>  #include "lacp.h"
>  #include "learn.h"
>  #include "mac-learning.h"
> +#include "meta-flow.h"
>  #include "multipath.h"
>  #include "netdev.h"
>  #include "netlink.h"
> @@ -4402,9 +4403,11 @@ static void
>  xlate_output_reg_action(struct action_xlate_ctx *ctx,
>                         const struct nx_action_output_reg *naor)
>  {
> +    struct mf_subfield src;
>     uint64_t ofp_port;
>
> -    ofp_port = nxm_read_field_bits(naor->src, naor->ofs_nbits, &ctx->flow);
> +    nxm_decode(&src, naor->src, naor->ofs_nbits);
> +    ofp_port = mf_get_subfield(&src, &ctx->flow);
>
>     if (ofp_port <= UINT16_MAX) {
>         xlate_output_action__(ctx, ofp_port, ntohs(naor->max_len));
> --
> 1.7.2.5
>
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