On Tue, Apr 28, 2026 at 7:33 AM Stefano Garzarella <[email protected]> wrote: > > On Mon, Apr 27, 2026 at 03:48:44PM -0700, Jorge Moreira wrote: > >On Mon, Apr 27, 2026 at 3:45 PM Stefan Hajnoczi <[email protected]> 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 <[email protected]> > >> > wrote: > >> > > > >> > > On Wed, 22 Apr 2026 at 03:16, Jorge Moreira <[email protected]> > >> > > wrote: > >> > > > > >> > > > On Tue, Apr 21, 2026 at 2:12 PM Stefan Hajnoczi > >> > > > <[email protected]> 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 > > Okay, but since, as we've seen, no frontend currently implements this, > we need to make it clear that a backend shouldn't necessarily expect the > kick injected from every frontend, but should support it in some way > since some of them can inject it. > > IMHO especially new backend implementations shouldn't rely on the kick > injection. > > So, to summarize: > - the frontend should also send a kick to restart the queues > - the backend should restart the queues after VHOST_USER_SET_VRING_KICK, > but it might also receive a kick > > >> 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. > > Yep, I think we are aligned. > > >> > >> 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? > > LGTM! > > > > >Sounds good to me > > > > @Jorge do you want to propose this change?
You have a much better idea of where and how this should be written, it will save us a few rounds of review if one of you makes that change. > > Thanks, > Stefano >
