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

From: Daniel P . Berrangé
Subject: Re: [PATCH 0/3] virtio-net: graceful drop of vhost for TAP
Date: Thu, 18 Feb 2021 09:30:36 +0000
User-agent: Mutt/2.0.5 (2021-01-21)

On Tue, Feb 09, 2021 at 10:04:30AM -0500, 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.

I should say the kernel's continual changing in CPU features that are
exposed has been responsible for a *huge* number of bugs with live
migration compatibility. libvirt, QEMU & apps have needed to introduce
a lot of extra code to try to cope with the changing CPU features across
migration and i still goes wrong to this very day, because we have to
migrate from prehistoric QEMU versions to quite modern versions.

IOW, the CPU features approach is a perfect example of why we should
*not* introduce a kernel dependancy in more areas of QEMU feature
enablement, and instead should strictly tie feature defaults to the
machine type versions.

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