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Re: [PATCH 0/3] virtio-net: graceful drop of vhost for TAP

From: Michael S. Tsirkin
Subject: Re: [PATCH 0/3] virtio-net: graceful drop of vhost for TAP
Date: Tue, 9 Feb 2021 10:04:30 -0500

On Tue, Feb 09, 2021 at 02:51:05PM +0000, Daniel P. Berrangé wrote:
> On Tue, Feb 09, 2021 at 09:34:20AM -0500, Michael S. Tsirkin wrote:
> > On Thu, Feb 04, 2021 at 10:29:12PM +0200, Yuri Benditovich wrote:
> > > This set of patches introduces graceful switch from tap-vhost to
> > > tap-no-vhost depending on guest features. Before that the features
> > > that vhost does not support were silently cleared in get_features.
> > > This creates potential problem of migration from the machine where
> > > some of virtio-net features are supported by the vhost kernel to the
> > > machine where they are not supported (packed ring as an example).
> > 
> > I still worry that adding new features will silently disable vhost for 
> > people.
> > Can we limit the change to when a VM is migrated in?
> Some management applications expect bi-directional live migration to
> work, so taking specific actions on incoming migration only feels
> dangerous. 

Could you be more specific?

Bi-directional migration is currently broken
when migrating new kernel->old kernel.

This seems to be the motivation for this patch, though I wish
it was spelled out more explicitly.

People don't complain much, but I'm fine with fixing that
with a userspace fallback.

I'd rather not force the fallback on others though: vhost is generally
specified explicitly by user while features are generally set
automatically, so this patch will make us override what user specified,
not nice.

> IMHO if the features we're adding cannot be expected to exist in
> host kernels in general, then the feature should defualt to off
> and require explicit user config to enable.
> Downstream distros which can guarantee newer kernels can flip the
> default in their custom machine types if they desire.
> Regards,
> Daniel

Unfortunately that will basically mean we are stuck with no new features
for years. We did what this patch is trying to change for years now, in
particular KVM also seems to happily disable CPU features not supported
by kernel so I wonder why we can't keep doing it, with tweaks for some
corner cases.

userspace and kernel not being in 100% sync wrt features is not
a corner case though, and switching backends seems like too big
a hammer.

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