Re: [Qemu-devel] [PATCH 2/2] virtio-net: mark VIRTIO_NET_F_GSO as legacy

[Halil Pasic <pasic@linux.vnet.ibm.com>]

On 11/07/2016 04:38 PM, Dr. David Alan Gilbert wrote:
> * Michael S. Tsirkin (mst@redhat.com) wrote:
>> > virtio 1.0 spec says this is a legacy feature bit,
>> > hide it from guests in legacy mode.
>> > 
>> > Note: for cross-version migration compatibility,
>> > we keep the bit set in host_features.
>> > The result will be that a guest migrating cross-version
>> > will see host features change under it.
>> > As guests only seem to read it once, this should
>> > not be an issue. Meanwhile, will work to fix guests to
>> > ignore this bit in virtio1 mode, too.
> OK, but if they're actually using the feature they'll carry
> on working ?
> 
> Dave
> 

The interesting scenario is when we migrate form older to newer
(otherwise the bugous situation can't emerge in the first place). Now
the change affects only what the guest sees when it reads host_features,
and that should happen only during the feature negotiation (once). That
means the change won't have any observable effect unless a reset
happens. And if we have a reset, then things are going to work out
correctly avoiding the bugous (but still working) state in both host and
guest. So yes, both will happily carry on using the feature, and carry
on working.

Halil