[ovs-dev] [PATCH] dpif-netdev: Check for PKT_RX_RSS_HASH flag.

[Flavio Leitner]

On Tue, Jun 16, 2015 at 07:39:00PM +0100, Daniele Di Proietto wrote:
> DPDK mbufs contain a valid RSS hash only if PKT_RX_RSS_HASH is
> set in 'ol_flags'.  Otherwise the hash is garbage and doesn't
> relate to the packet.
> 
> This fixes an issue with vhost, which, being a virtual NIC, doesn't
> compute the hash.
> 
> Unfortunately the ixgbe vPMD doesn't set the PKT_RX_RSS_HASH, forcing
> OVS to compute an hash is software.  This has a significant impact on
> performance (-30% throughput in a single flow setup) which can be
> mitigated in the CPU supports crc32c instructions.
> 
> Reported-by: Dongjun <dongj at dtdream.com>
> Suggested-by: Flavio Leitner <fbl at sysclose.org>
> Signed-off-by: Daniele Di Proietto <diproiettod at vmware.com>
> ---
>  lib/dp-packet.h   | 11 +++++++++++
>  lib/dpif-netdev.c |  2 +-
>  2 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/dp-packet.h b/lib/dp-packet.h
> index e4c2593..6840750 100644
> --- a/lib/dp-packet.h
> +++ b/lib/dp-packet.h
> @@ -529,11 +529,22 @@ dp_packet_set_rss_hash(struct dp_packet *p, uint32_t hash)
>  {
>  #ifdef DPDK_NETDEV
>      p->mbuf.hash.rss = hash;
> +    p->mbuf.ol_flags |= PKT_RX_RSS_HASH;
>  #else
>      p->rss_hash = hash;
>  #endif
>  }
>  
> +static inline bool
> +dp_packet_rss_valid(struct dp_packet *p)
> +{
> +#ifdef DPDK_NETDEV
> +    return p->mbuf.ol_flags & PKT_RX_RSS_HASH;
> +#else
> +    return true;
> +#endif
> +}

The above triggers the following when not using dpdk.
lib/dp-packet.h: In function 'dp_packet_rss_valid':
lib/dp-packet.h:539:39: warning: unused parameter 'p'
[-Wunused-parameter]

fbl