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[87.11.6.2]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48a7b92adefsm30673255e9.3.2026.04.29.07.26.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 29 Apr 2026 07:26:32 -0700 (PDT) Date: Wed, 29 Apr 2026 16:26:24 +0200 From: Stefano Garzarella To: Jorge Moreira Cc: Stefan Hajnoczi , hreitz@redhat.com, gmaglione@redhat.com, "Michael S . Tsirkin" , Hanna Czenczek , Pierrick Bouvier , qemu-devel@nongnu.org Subject: Re: [PATCH] vhost-user.rst: Explicitly allow front-end to write to kick FDs Message-ID: References: <20260420181818.GC405461@fedora> <20260421211237.GC466778@fedora> <20260427224545.GH218226@fedora> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Received-SPF: pass client-ip=170.10.133.124; envelope-from=sgarzare@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Apr 28, 2026 at 10:19:30AM -0700, Jorge Moreira wrote: >On Tue, Apr 28, 2026 at 7:33 AM Stefano Garzarella 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 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 wrote: >> >> > > >> >> > > On Wed, 22 Apr 2026 at 03:16, Jorge Moreira wrote: >> >> > > > >> >> > > > On Tue, Apr 21, 2026 at 2:12 PM Stefan Hajnoczi 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. Okay, I can try, but next week. Thanks, Stefano