Re: [PATCH] kvm tools: virtio-net mergable rx buffers

[Sasha Levin <sasha.levin@oracle.com>]

On 04/28/2013 08:44 PM, Rusty Russell wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
>> > On Wed, Apr 24, 2013 at 09:51:57AM +0300, Pekka Enberg wrote:
>>> >> Hi,
>>> >> 
>>> >> On 04/23/2013 12:35 PM, Eric Northup wrote:
>>>>> >> >> Do you care about guests with drivers that don't negotiate
>>>>> >> >> VIRTIO_NET_F_MRG_RXBUF?
>>> >> 
>>> >> On Wed, Apr 24, 2013 at 5:51 AM, Sasha Levin <sasha.levin@oracle.com> wrote:
>>>> >> > We usually try to keep backward compatibility, but in this case
>>>> >> > mergable RX buffers are about 5 years old now, so it's safe to
>>>> >> > assume they'll be running in any guest.
>>>> >> >
>>>> >> > Unless there is a specific reason to allow working without them
>>>> >> > I'd rather keep the code simple in this case.
>>> >> 
>>> >> Are there such guests around? What's the failure scenario for them
>>> >> after this patch?
>>> >> 
>>> >>                         Pekka
>> >
>> > Warning: have not looked at the patch, just a general comment.
>> >
>> > I think it's reasonable to assume embedded guests such as PXE won't
>> > negotiate any features.  And, running old guests is one of the reasons
>> > people use virtualization at all. So 5 years is not a lot.
>> >
>> > In any case, stick to the device spec please, if you want it changed
>> > please send a spec patch, don't deviate from it randomly.
> Supporting old guests is an quality of implementation issue.  It's like
> any ABI: if noone will notice, you can remove stuff.
> 
> But the case of "I can receive GSO packets but I don't support mergeable
> buffers" is a trivial one: you can "support" it by pretending the guest
> can't handle GSO :)
> 
> If you want to support non-Linux guests (eg. bootloaders), you probably
> want to keep support for very dumb drivers with no mergable rxbufs
> though.

Yup, I'm planning on sending a version that supports older guests soonish.


Thanks,
Sasha