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

[Michael S. Tsirkin]

On Fri, Feb 05, 2021 at 08:38:49AM -0500, 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.
> 
> Also, let's document this behaviour with the vhost option so people
> are not suprized.

Here's another thing that bothers me.

At the moment we easily add new features, enabled by default,
as long as kernels are consistent on source and destination
everything works fine.

With this patch first time we add a new feature that kernel
does not support, vhost gets disabled. Given lots of people
update their kernels less frequently than userspace,
lots of users will start running with vhost off all of a sudden.

Don't have good suggestions yet.

-- 
MST