From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists1p.gnu.org (lists1p.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 95AECFF8864 for ; Mon, 27 Apr 2026 22:47:02 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1wHUiJ-0003y3-JS; Mon, 27 Apr 2026 18:46:03 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wHUiI-0003xg-Kj for qemu-devel@nongnu.org; Mon, 27 Apr 2026 18:46:02 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wHUiC-0005GE-4h for qemu-devel@nongnu.org; Mon, 27 Apr 2026 18:46:01 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1777329954; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=KY+Qxr4Xuu4B/qsQnNyEQkfKF5LUndllTW761FqOerg=; b=VA2nUOx3yCt9mRmi20N9sOqUXrSxV0KmExe0enPI/IpaBNi1nb4x6p/DfReCJnPrpEK+0N aA6c66joxbAJDWi09PUCOwhw7mpCSecO0WkKK5fons8140/VR0U5Gkq5c+5h96hXtc29al 06Ci9Q0AbnDFzxBbSCqtP3uc9LLCW8g= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-380-0VynG02dNASKyydY448d6A-1; Mon, 27 Apr 2026 18:45:51 -0400 X-MC-Unique: 0VynG02dNASKyydY448d6A-1 X-Mimecast-MFC-AGG-ID: 0VynG02dNASKyydY448d6A_1777329950 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id AD28818003FC; Mon, 27 Apr 2026 22:45:49 +0000 (UTC) Received: from localhost (unknown [10.44.48.31]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 1334D300070A; Mon, 27 Apr 2026 22:45:47 +0000 (UTC) Date: Mon, 27 Apr 2026 18:45:45 -0400 From: Stefan Hajnoczi To: Jorge Moreira Cc: Stefano Garzarella , 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: <20260427224545.GH218226@fedora> References: <20260411021205.3592118-1-jemoreira@google.com> <20260420181818.GC405461@fedora> <20260421211237.GC466778@fedora> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="woZ49ArodzuJ24yY" Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@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 --woZ49ArodzuJ24yY Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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 wrot= e: > > > > > > 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 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 > > >=20 > 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_ty= pe=3Dheads#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 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 --woZ49ArodzuJ24yY Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQEzBAEBCgAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmnv5xkACgkQnKSrs4Gr c8gwnwf/dVH0le+pTq7BbMCkB4E4a+YwsKleNNIn/eO4lviE0pP2VPriuES38VKo UidbQ0+4StxArA6Rq0/qe2BEr5hTeBgzfQRRnaNpf18aZ8sOpzCRnGZ4oTk+3/pI wOvPCme59bY7RNXS/GnNUSG9rEKsF6AVKOdXu/p+pr4bLuuNEe0Euj+N7zwMnzJc DPbAoLo0fanHX0XN1toca/8au+mLKHEXwFase33wXY/0S6W5kI1akvG73jp/f1tj Vad3B/Uek4O82+QSn5OU3EVbd5oMsjRZ45MQ62q+6vD19ObWb/3/YLwKPBZlB9wi AQuZMI7FbKvwZLRmZfgwdv0zPVwLFQ== =cr/Z -----END PGP SIGNATURE----- --woZ49ArodzuJ24yY--