On Wed, Apr 22, 2026 at 12:20:52PM -0700, Jorge Moreira wrote:
> On Wed, Apr 22, 2026 at 1:32 AM Stefano Garzarella <sgarzare@redhat.com> wrote:
> >
> > On Wed, 22 Apr 2026 at 03:16, Jorge Moreira <jemoreira@google.com> wrote:
> > >
> > > On Tue, Apr 21, 2026 at 2:12 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > > >
> > > > On Mon, Apr 20, 2026 at 05:48:13PM -0700, Jorge Moreira wrote:
> > > > > While starting the vrings on SET_VRING_KICK could solve the state
> > > > > machine issue, it still won't notify the back-end that buffers are
> > > > > ready (the driver won't do this). Non-polling back-ends depend on this
> > > > > kick, especially for queues where data flows only from the driver to
> > > > > the back-end. Most implementations likely attempt to read from the
> > > > > queue only after receiving the kick.
> > > >
> > > > This is an interesting question to clarify in the spec.
> >
> > Yep, which is in part related to what I wrote in the other reply:
> > "I think the main issue to clarify is what the device should do
> > when the vrings are configured, but the driver has already been
> > initialized (which is usually the case after migration)."
> >
> > > >
> > > > Stefan
> > >
> > > This is the question that interests me most, to be honest. I'd rather
> > > have the discussion about when to activate the vrings in a different
> > > thread and keep this one focused on whether the front-end should send
> > > the kick or if the back-end is expected to check if there are "new"
> > > buffers in the vring after restore.
> > >
> >
> > IMO we don't need anything from the VMM. When the device receives
> > SET_VRING_KICK, it can check if the vring already contains buffers
> > (and this is the part we might need to clarify) and wake-up the other
> > threads (or always wake-ups them, as crosvm does IIUC, and let them
> > perform this check).
> > After sending the SET_VRING_KICK message to the device, the VMM has
> > the exact same knowledge of the vring state as the device, therefore,
> > it's still unclear to me why we need to inject that kick.
> >
> > Stefano
> >
> 
> Is it possible to activate a vring after it has been deactivated with
> VHOST_USER_GET_VRING_BASE? If yes, does the front-end need to send the
> kick file descriptor again with VHOST_USER_SET_VRING_KICK to
> reactivate it?

Hi Jorge and Stefano,
Yes, VHOST_USER_GET_VRING_BASE -> VHOST_USER_SET_VRING_KICK occurs when
a VM is paused and then resumed.

You can stress test this by driving I/O using iperf (virtio-net) or fio
(virtio-blk) inside the guest and sending 'stop'/'cont' commands to
QEMU's monitor.

Here is QEMU's code for starting (including re-starting) rings:
https://gitlab.com/qemu-project/qemu/-/blob/master/hw/virtio/vhost.c?ref_type=heads#L1341

QEMU does not inject a kick. The back-end must check the rings itself.

I'm not sure that all vhost-user back-ends actually check the rings. I
think back-ends should do it, but we should also update the spec with an
front-end implementation note recommending injecting a kick after
VHOST_USER_SET_VRING_KICK completes in order to maximize compatibility
with implementations that follow the current spec wording. And at the
same time I think the spec should also be changed to say that
VHOST_USER_SET_VRING_KICK starts the ring and back-ends SHOULD check the
vring upon processing the message.

That seems like it would clean up the issues without introducing
compatibility issues or making existing implementations non-compliant
with the updated spec.

What do you think?

Stefan