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 <[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 would worry that there could be buggy drivers that expect buffers not consumed until the 1st kick. 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. > 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
