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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: <20260429155548.GB12211@fedora> References: <20260420181818.GC405461@fedora> <20260421211237.GC466778@fedora> <20260427224545.GH218226@fedora> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="Do/ZaTeJ5plpynVu" Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.129.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 12 X-Spam_score: 1.2 X-Spam_bar: + X-Spam_report: (1.2 / 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_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no 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 --Do/ZaTeJ5plpynVu Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Apr 28, 2026 at 04:33:23PM +0200, Stefano Garzarella wrote: > On Mon, Apr 27, 2026 at 03:48:44PM -0700, Jorge Moreira wrote: > > On Mon, Apr 27, 2026 at 3:45=E2=80=AFPM Stefan Hajnoczi wrote: > > >=20 > > > On Wed, Apr 22, 2026 at 12:20:52PM -0700, Jorge Moreira wrote: > > > > On Wed, Apr 22, 2026 at 1:32=E2=80=AFAM Stefano Garzarella wrote: > > > > > > > > > > On Wed, 22 Apr 2026 at 03:16, Jorge Moreira wrote: > > > > > > > > > > > > On Tue, Apr 21, 2026 at 2:12=E2=80=AFPM 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 buff= ers are > > > > > > > > ready (the driver won't do this). Non-polling back-ends dep= end on this > > > > > > > > kick, especially for queues where data flows only from the = driver to > > > > > > > > the back-end. Most implementations likely attempt to read f= rom 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 diff= erent > > > > > > thread and keep this one focused on whether the front-end shoul= d 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 o= ther > > > > > threads (or always wake-ups them, as crosvm does IIUC, and let th= em > > > > > perform this check). > > > > > After sending the SET_VRING_KICK message to the device, the VMM h= as > > > > > the exact same knowledge of the vring state as the device, theref= ore, > > > > > 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 wi= th > > > > 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? > > >=20 > > > Hi Jorge and Stefano, > > > Yes, VHOST_USER_GET_VRING_BASE -> VHOST_USER_SET_VRING_KICK occurs wh= en > > > a VM is paused and then resumed. > > >=20 > > > You can stress test this by driving I/O using iperf (virtio-net) or f= io > > > (virtio-blk) inside the guest and sending 'stop'/'cont' commands to > > > QEMU's monitor. > > >=20 > > > 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=3Dheads#L1341 > > >=20 > > > QEMU does not inject a kick. The back-end must check the rings itself. > > >=20 > > > 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 >=20 > 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. >=20 > IMHO especially new backend implementations shouldn't rely on the kick > injection. >=20 > 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 Yes, agreed. >=20 > > > 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. >=20 > Yep, I think we are aligned. >=20 > > >=20 > > > That seems like it would clean up the issues without introducing > > > compatibility issues or making existing implementations non-compliant > > > with the updated spec. > > >=20 > > > What do you think? >=20 > LGTM! >=20 > >=20 > > Sounds good to me > >=20 >=20 > @Jorge do you want to propose this change? >=20 > Thanks, > Stefano >=20 --Do/ZaTeJ5plpynVu Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQEzBAEBCgAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmnyKgQACgkQnKSrs4Gr c8irOgf9GIG6raSpw8ir4XanQzXBrtT6n/GEyWSmSW7ap8SIGhKi3yYvUSaSjkkA h8bDyMk6X0qYRXMSTTBFvTH8pL8dzaZ+0AzwxNhKareRmEKRuU+Zyb1tqqfdM5n+ LUudcI3Bg3IEs3EBAMBAIPFVjfKSeOh/AOgURFI6aqwm0xNpoR03UVB7DGmNWGHG 6Yf3i14AvvRLmNWi/F0mLmnLAmlYZ7Qbs7i9o7Ze+irKcZlzbpa+sU/pisvKLgnD 2cXSeE6mHTBnnQkQz+8EhXMglNFTe+facm8pPcEJ0hvxyKtE23O4hs6DhyAtDQLD PFZ+pVNc2vl5kY9cGDtG67iY4ChfRA== =Qnpg -----END PGP SIGNATURE----- --Do/ZaTeJ5plpynVu--