Re: [Virtio-fs] [PATCH v3 0/5] vhost-user: Back-end state migration

[Stefan Hajnoczi]

On Wed, Sep 27, 2023 at 10:32:14AM +0200, Hanna Czenczek wrote:
> On 26.09.23 22:10, Stefan Hajnoczi wrote:
> > Hi Hanna,
> > I was thinking about how this could work without SUSPEND/RESUME. What
> > do you think of the following?
> > 
> > 1. The front-end sends VHOST_USER_RESET_DEVICE (or
> > VHOST_USER_RESET_OWNER, when necessary) when the guest driver resets
> > the device but not on vhost_dev_start()/vhost_dev_stop().
> 
> This is half the work of SUSPEND/RESUME.  It isn’t easy to do.

I sent a patch series to bring VHOST_USER_RESET_DEVICE to all vhost-user
devices:
20230927192737.528280-1-stefanha@redhat.com/T/#t">https://lore.kernel.org/qemu-devel/20230927192737.528280-1-stefanha@redhat.com/T/#t

> 
> > 2. Suspend the device when all virtqueues are stopped via
> > VHOST_USER_GET_VRING_BASE. Resume the device after at least one
> > virtqueue is started and enabled.
> > 3. Ignore VHOST_USER_SET_STATUS.
> > 
> > Reset would work. The device would suspend and resume without losing
> > state. Existing vhost-user backends already behave like this in
> > practice (they often don't implement RESET_DEVICE).
> 
> I don’t understand the point, though.  Today, reset in practice is a no-op
> anyway, precisely because we only send SET_STATUS 0, don’t fall back to
> RESET_OWNER/RESET_DEVICE, and no back-end implements SET_STATUS 0 as a
> reset.  By sending RESET_* in case of a guest-initiated reset and nothing in
> case of stop/cont, we effectively don’t change anything about the latter
> (which is what SUSPEND/RESUME would be for), but only fix the former case. 
> While I agree that it’s wrong that we don’t really reset the back-end in
> case of a guest-initiated reset, this is the first time in this whole
> discussion that that part has been presented as a problem that needs fixing
> now.
> 
> So the proposal effectively changes nothing for the vhost_dev_stop()/start()
> case where we’d want to make use of SUSPEND/RESUME, but only for the case
> where we would not use it.

We discussed this on a call today. 2 & 3 are additions to the spec that
Hanna has agreed to work on.

Stefan

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