From nobody Thu Nov 14 16:23:12 2024 Delivered-To: importer@patchew.org Authentication-Results: mx.zohomail.com; dkim=pass; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=pass(p=none dis=none) header.from=redhat.com ARC-Seal: i=1; a=rsa-sha256; t=1696278864; cv=none; d=zohomail.com; s=zohoarc; b=mifIOQXfSL5LABzyWN5dFPg0KttAiMQfXBUifVs1POMEHQZAff+RxylsK8qWXmm1V5qjDL6zxfWs46zKWil4SExgQuYAfOZnqalGAFlVUe9Hy8pnUXyq1aF/J73Dru9bjXXe4Os9YDQgbf4Bg4bkmR0zF90mt+uFmIeWudzO6WE= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=zohomail.com; s=zohoarc; t=1696278864; h=Content-Type:Content-Transfer-Encoding:Cc:Date:From:In-Reply-To:List-Subscribe:List-Post:List-Id:List-Archive:List-Help:List-Unsubscribe:MIME-Version:Message-ID:References:Sender:Subject:To; bh=BhFdydZThHY12w/bm97zGukl7AatGfEDNNzJZgKLQ8M=; b=NDfv/+WDiJYqKoEXfr3ugb+/Sf8NSPRTqdxdNaF2YTHqgoTC4bxJ6aQdbYoCj8kIltAptVj/tav6cx2Bjsl0Kdme2gPp6uBh6p5ttRUtsGALy65zrZbhxqw8Q/KN5Cm56kSXs8qhfxqMciUnVA3QTgTJDVajJoU2R9S8LHLhVtk= ARC-Authentication-Results: i=1; mx.zohomail.com; dkim=pass; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=pass header.from= (p=none dis=none) Return-Path: Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) by mx.zohomail.com with SMTPS id 1696278864607741.8743745212322; Mon, 2 Oct 2023 13:34:24 -0700 (PDT) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qnPb5-0003Xg-0F; Mon, 02 Oct 2023 16:32:55 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qnPb3-0003Vo-Lw for qemu-devel@nongnu.org; Mon, 02 Oct 2023 16:32:53 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qnPb1-0001Vo-Iu for qemu-devel@nongnu.org; Mon, 02 Oct 2023 16:32:53 -0400 Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-639-wn5rVc1bOtmrUloAqPTxPg-1; Mon, 02 Oct 2023 16:32:47 -0400 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 45023101A590; Mon, 2 Oct 2023 20:32:47 +0000 (UTC) Received: from lacos-laptop-9.usersys.redhat.com (unknown [10.39.192.119]) by smtp.corp.redhat.com (Postfix) with ESMTP id 658FE40C6EBF; Mon, 2 Oct 2023 20:32:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1696278770; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=BhFdydZThHY12w/bm97zGukl7AatGfEDNNzJZgKLQ8M=; b=GdQXHtndT6dHBAJZEyVIe3ViFvOIBc7r0ijVRyViv+Xz59GBBpD2AWDMHrN3cQ6eQFbzOB MLTxj0VyYedZL4zTU2XfEbpAg/AU4hZfTqAaj4xieLB8KR+6oOlQOvTgXYhPmHFOkv+ode Y3hWCp0JUT2DdPRAlWV3GJYpezsUUwk= X-MC-Unique: wn5rVc1bOtmrUloAqPTxPg-1 From: Laszlo Ersek To: qemu-devel@nongnu.org, lersek@redhat.com Cc: "Michael S. Tsirkin" , Eugenio Perez Martin , German Maglione , Liu Jiang , Sergio Lopez Pascual , Stefano Garzarella Subject: [PATCH v3 7/7] vhost-user: call VHOST_USER_SET_VRING_ENABLE synchronously Date: Mon, 2 Oct 2023 22:32:21 +0200 Message-Id: <20231002203221.17241-8-lersek@redhat.com> In-Reply-To: <20231002203221.17241-1-lersek@redhat.com> References: <20231002203221.17241-1-lersek@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.2 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.129.124; envelope-from=lersek@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_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=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: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1696278866795100003 Content-Type: text/plain; charset="utf-8" (1) The virtio-1.2 specification writes: > 3 General Initialization And Device Operation > 3.1 Device Initialization > 3.1.1 Driver Requirements: Device Initialization > > [...] > > 7. Perform device-specific setup, including discovery of virtqueues for > the device, optional per-bus setup, reading and possibly writing the > device=E2=80=99s virtio configuration space, and population of virtque= ues. > > 8. Set the DRIVER_OK status bit. At this point the device is =E2=80=9Cliv= e=E2=80=9D. and > 4 Virtio Transport Options > 4.1 Virtio Over PCI Bus > 4.1.4 Virtio Structure PCI Capabilities > 4.1.4.3 Common configuration structure layout > 4.1.4.3.2 Driver Requirements: Common configuration structure layout > > [...] > > The driver MUST configure the other virtqueue fields before enabling the > virtqueue with queue_enable. > > [...] (The same statements are present in virtio-1.0 identically, at .) These together mean that the following sub-sequence of steps is valid for a virtio-1.0 guest driver: (1.1) set "queue_enable" for the needed queues as the final part of device initialization step (7), (1.2) set DRIVER_OK in step (8), (1.3) immediately start sending virtio requests to the device. (2) When vhost-user is enabled, and the VHOST_USER_F_PROTOCOL_FEATURES special virtio feature is negotiated, then virtio rings start in disabled state, according to . In this case, explicit VHOST_USER_SET_VRING_ENABLE messages are needed for enabling vrings. Therefore setting "queue_enable" from the guest (1.1) -- which is technically "buffered" on the QEMU side until the guest sets DRIVER_OK (1.2) -- is a *control plane* operation, which -- after (1.2) -- travels from the guest through QEMU to the vhost-user backend, using a unix domain socket. Whereas sending a virtio request (1.3) is a *data plane* operation, which evades QEMU -- it travels from guest to the vhost-user backend via eventfd. This means that operations ((1.1) + (1.2)) and (1.3) travel through different channels, and their relative order can be reversed, as perceived by the vhost-user backend. That's exactly what happens when OVMF's virtiofs driver (VirtioFsDxe) runs against the Rust-language virtiofsd version 1.7.2. (Which uses version 0.10.1 of the vhost-user-backend crate, and version 0.8.1 of the vhost crate.) Namely, when VirtioFsDxe binds a virtiofs device, it goes through the device initialization steps (i.e., control plane operations), and immediately sends a FUSE_INIT request too (i.e., performs a data plane operation). In the Rust-language virtiofsd, this creates a race between two components that run *concurrently*, i.e., in different threads or processes: - Control plane, handling vhost-user protocol messages: The "VhostUserSlaveReqHandlerMut::set_vring_enable" method [crates/vhost-user-backend/src/handler.rs] handles VHOST_USER_SET_VRING_ENABLE messages, and updates each vring's "enabled" flag according to the message processed. - Data plane, handling virtio / FUSE requests: The "VringEpollHandler::handle_event" method [crates/vhost-user-backend/src/event_loop.rs] handles the incoming virtio / FUSE request, consuming the virtio kick at the same time. If the vring's "enabled" flag is set, the virtio / FUSE request is processed genuinely. If the vring's "enabled" flag is clear, then the virtio / FUSE request is discarded. Note that OVMF enables the queue *first*, and sends FUSE_INIT *second*. However, if the data plane processor in virtiofsd wins the race, then it sees the FUSE_INIT *before* the control plane processor took notice of VHOST_USER_SET_VRING_ENABLE and green-lit the queue for the data plane processor. Therefore the latter drops FUSE_INIT on the floor, and goes back to waiting for further virtio / FUSE requests with epoll_wait. Meanwhile OVMF is stuck waiting for the FUSET_INIT response -- a deadlock. The deadlock is not deterministic. OVMF hangs infrequently during first boot. However, OVMF hangs almost certainly during reboots from the UEFI shell. The race can be "reliably masked" by inserting a very small delay -- a single debug message -- at the top of "VringEpollHandler::handle_event", i.e., just before the data plane processor checks the "enabled" field of the vring. That delay suffices for the control plane processor to act upon VHOST_USER_SET_VRING_ENABLE. We can deterministically prevent the race in QEMU, by blocking OVMF inside step (1.2) -- i.e., in the write to the device status register that "unleashes" queue enablement -- until VHOST_USER_SET_VRING_ENABLE actually *completes*. That way OVMF's VCPU cannot advance to the FUSE_INIT submission before virtiofsd's control plane processor takes notice of the queue being enabled. Wait for VHOST_USER_SET_VRING_ENABLE completion by: - setting the NEED_REPLY flag on VHOST_USER_SET_VRING_ENABLE, and waiting for the reply, if the VHOST_USER_PROTOCOL_F_REPLY_ACK vhost-user feature has been negotiated, or - performing a separate VHOST_USER_GET_FEATURES *exchange*, which requires a backend response regardless of VHOST_USER_PROTOCOL_F_REPLY_ACK. Cc: "Michael S. Tsirkin" (supporter:vhost) Cc: Eugenio Perez Martin Cc: German Maglione Cc: Liu Jiang Cc: Sergio Lopez Pascual Cc: Stefano Garzarella Signed-off-by: Laszlo Ersek Reviewed-by: Stefano Garzarella Tested-by: Albert Esteve [lersek@redhat.com: work Eugenio's explanation into the commit message, about QEMU containing step (1.1) until step (1.2)] Reviewed-by: Eugenio P=C3=A9rez --- Notes: v3: =20 - pick up R-b from Eugenio, T-b from Albert =20 - clarify commit message (also give permanent credit for the clarification; I feel the change is important enough) [Eugenio] =20 v2: =20 - pick up R-b from Stefano =20 - update virtio spec reference from 1.0 to 1.2 (also keep the 1.0 ref) in the commit message; re-check the quotes / section headers [Stefano] =20 - summarize commit message in code comment [Stefano] hw/virtio/vhost-user.c | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-) diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c index 18e15a9bb359..41842eb023b5 100644 --- a/hw/virtio/vhost-user.c +++ b/hw/virtio/vhost-user.c @@ -1235,7 +1235,21 @@ static int vhost_user_set_vring_enable(struct vhost_= dev *dev, int enable) .num =3D enable, }; =20 - ret =3D vhost_set_vring(dev, VHOST_USER_SET_VRING_ENABLE, &state, = false); + /* + * SET_VRING_ENABLE travels from guest to QEMU to vhost-user backe= nd / + * control plane thread via unix domain socket. Virtio requests tr= avel + * from guest to vhost-user backend / data plane thread via eventf= d. + * Even if the guest enables the ring first, and pushes its first = virtio + * request second (conforming to the virtio spec), the data plane = thread + * in the backend may see the virtio request before the control pl= ane + * thread sees the queue enablement. This causes (in fact, require= s) the + * data plane thread to discard the virtio request (it arrived on a + * seemingly disabled queue). To prevent this out-of-order deliver= y, + * don't let the guest proceed to pushing the virtio request until= the + * backend control plane acknowledges enabling the queue -- IOW, p= ass + * wait_for_reply=3Dtrue below. + */ + ret =3D vhost_set_vring(dev, VHOST_USER_SET_VRING_ENABLE, &state, = true); if (ret < 0) { /* * Restoring the previous state is likely infeasible, as well = as