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

[Daniel P . Berrangé]

On Wed, Feb 10, 2021 at 02:19:59PM +0800, Jason Wang wrote:
> 
> On 2021/2/9 下午11:04, Michael S. Tsirkin wrote:
> > 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.
> 
> 
> It's probably not the corner case.
> 
> So my understanding is when a feature is turned on via command line, it
> should not be cleared silently otherwise we may break migration for sure.
> 
> E.g when packed=on is specified, we should disable vhost instead of clear it
> from the device.

If something is explicitly turned on by the user, they expect that feature
to be honoured, or an error to be raised.

If something is not explicitly turned on by the user, the behaviour wrt the
default should be stable for any given machine type version.

IOW, if you disable vhost by default when packed=on is set, then you can't
later switch to letting vhost be enabled with packed=on, unless you tie
that change to a new machine type.

If the user has explicitly said  packed=on *and* vhost=on, then should
must honour that, or raise an error if the combination is unsupportable.
Silently disabling vhost, then vhost=on is not ok.

Regards,
Daniel
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