Re: [PATCH 3/3] virtio-net: graceful fallback to vhost=off for tap netdev

[Yuri Benditovich]

On Mon, Feb 8, 2021 at 5:15 AM Jason Wang <jasowang@redhat.com> wrote:
>
>
> On 2021/2/5 下午9:38, Michael S. Tsirkin wrote:
> > On Thu, Feb 04, 2021 at 10:29:15PM +0200, Yuri Benditovich wrote:
> >> Currently virtio-net silently clears features if they are
> >> not supported by respective vhost. This may create migration
> >> problems in future if vhost features on the source and destination
> >> are different. Implement graceful fallback to no-vhost mode
> >> when some acked features contradict with vhost. The decision is
> >> taken on set_features call and the vhost will be disabled
> >> till next reset (or migration).
> >> Such fallback is currently enabled only for TAP netdev.
> >>
> >> Signed-off-by: Yuri Benditovich<yuri.benditovich@daynix.com>
> > Sounds good, but I don't think we should do this if
> > vhostforce=on is set.
>
>
> If we do this, does it mean we won't maintain migration compatibility
> when vhostforce is on?

AFAIU, the 'vhostforce=on' should mean the vhost can't be disabled (if
I'm not mistaken this is typically used for vhost-user).
So we can view this case as similar to vhost-vdpa and vhost-user.

>
> Thanks
>
>
> >
> > Also, let's document this behaviour with the vhost option so people
> > are not suprized.
> >
>