[ovs-dev] [RFC] ofp-actions: Remove expectation of struct ofpact size.

[Ben Pfaff]

On Mon, Mar 10, 2014 at 02:12:27PM -0700, Gurucharan Shetty wrote:
> struct ofpact has enums that are packed in case of __GNUC__.
> This packing does not occur for visual studio. For 'struct ofpact_nest',
> we are expecting that this structure is less than or equal to 8 bytes
> which won't be true in compilers where enums are not packed.
> 
> Signed-off-by: Gurucharan Shetty <gshetty at nicira.com>
> ---
> There must have been a reason why there was this BUILD_ASSERT in the
> first place. But, I do not understand the reason. (Unit tests don't
> fail after removing the OVS_PACKED_ENUM macro from the related enums
> and this build assert.)

The goal of OFPACT_ALIGNTO is to make sure that every ofpact starts on
an 8-byte boundary, which is in turn important because RISC machines
often require 8-byte alignment for e.g. pointers.  Since ofpact_nest
contains nested ofpacts, the ofpact[] array inside it needs to be on
an 8-byte boundary too.  The build assertion checked for that, but it
was too strict: any multiple of 8 bytes will work, but it required
exactly 8 bytes.

I think that the build assertion should be relaxed from "<offset> ==
OFPACT_ALIGNTO", to "<offset> % OFPACT_ALIGNTO == 0".

Does that make sense?

Thanks,

Ben.