Re: [PATCH] vhost-user.rst: Explicitly allow front-end to write to kick FDs

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Apr 28, 2026 at 02:59:58AM -0400, Michael S. Tsirkin wrote:
> On Mon, Apr 27, 2026 at 06:45:45PM -0400, Stefan Hajnoczi wrote:
> > 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 would worry that there could be buggy drivers that expect buffers not
> consumed until the 1st kick.

This describes the behavior today. I'm not sure whether you are
commenting on the paragraph I wrote below suggesting how to change
things or whether you are commenting on the current state?

> A safer thing to do would be for source to record the fact there
> was a kick after reset and for destination to then inject/check.

Can you rephrase that, it's unclear to me what "source" and
"destination" mean?

Thanks,
Stefan

> > 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
> 
> 

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