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 lists.gnu.org (lists.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 498A6E7AD5E for ; Tue, 3 Oct 2023 14:41:53 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qngac-0001j6-Nm; Tue, 03 Oct 2023 10:41:34 -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 1qngaZ-0001iF-SW for qemu-devel@nongnu.org; Tue, 03 Oct 2023 10:41:32 -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 1qngaU-0000zx-Kl for qemu-devel@nongnu.org; Tue, 03 Oct 2023 10:41:31 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1696344085; 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=9Ukgq7BfGKvmiAb1h1EQeoTLNqvwUUzsYFbXYYRdZFE=; b=S5r8WNpRrpw0i1NLcJpNJOCaqMCFpH5G7AueCE/UJwYxsYtK8WjEcv9cWmh4fnHglNGzcr 53z25LpAOYhum4f0bSeWzHl5dB4+/qs8dWlBqUlfPK0KiH9p4l+7c+UbFN2rbEr1YUlPyd X5c5HkqZy5MIrhkqiQ8ZUkn1BR+koJk= Received: from mail-wr1-f69.google.com (mail-wr1-f69.google.com [209.85.221.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-489-YrhY8itmN5yEUg5-_PsCXg-1; Tue, 03 Oct 2023 10:41:24 -0400 X-MC-Unique: YrhY8itmN5yEUg5-_PsCXg-1 Received: by mail-wr1-f69.google.com with SMTP id ffacd0b85a97d-326f05ed8f9so814215f8f.0 for ; Tue, 03 Oct 2023 07:41:24 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1696344083; x=1696948883; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:from:date :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=9Ukgq7BfGKvmiAb1h1EQeoTLNqvwUUzsYFbXYYRdZFE=; b=lBfEvCQ2oJt7xMt9goI2cFU4+zZJFS5ulBWEZc0n5Wn/WFwpfne/7kNnCOocytyDD+ 1nkN5rBJ2sWNK9iy28Yh0K4+CMa5YTYlFbneeIJznkvxlBr2eZjIPepITphIgFiK4gsI w/xPFuXRjuo5lLuF6LWcHhgyU5b0NMvMph73BW8VOGGc3Blr2OW39QKlYaz2HcwlM7bc ac4WBc3SbRgPifWSgC3/mmHEdTjbm87MmxULYpCh7oq9Qh9UjQkyLdND1EgStG2chb5W JTH29sxGZMeAQWxlFi5FTAwVxKchs0Na2JdQHJiM8DWaEvCGpHplIwsGnwO10P0h0uQH SIkQ== X-Gm-Message-State: AOJu0YwqNJktpp5QEa0fU90PXeqBLgfLjv5DhvHhw5NVT+ao+3fGawJ1 t9/gvpasusi98i0LERJJVPmAlBV8+W+a64zsUYmN4epJFgP8PqTnGJYTLYrSFljschhaqm7W7CR qMhYbTGamet0jaWxLdj7pFmQ= X-Received: by 2002:a5d:45c9:0:b0:31f:f326:766b with SMTP id b9-20020a5d45c9000000b0031ff326766bmr12449287wrs.6.1696344082756; Tue, 03 Oct 2023 07:41:22 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEGkc9tkGOal62Mg8utiGr8xosGaBWFNYzIKEVRnNBpIleahCS4Sj13UzF8t8ueVwhX2UQ3qw== X-Received: by 2002:a5d:45c9:0:b0:31f:f326:766b with SMTP id b9-20020a5d45c9000000b0031ff326766bmr12449275wrs.6.1696344082402; Tue, 03 Oct 2023 07:41:22 -0700 (PDT) Received: from redhat.com ([2.52.132.27]) by smtp.gmail.com with ESMTPSA id k16-20020a5d4290000000b00325b29a6441sm1723351wrq.82.2023.10.03.07.41.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 03 Oct 2023 07:41:21 -0700 (PDT) Date: Tue, 3 Oct 2023 10:41:16 -0400 From: "Michael S. Tsirkin" To: Laszlo Ersek Cc: qemu-devel@nongnu.org, Eugenio Perez Martin , German Maglione , Liu Jiang , Sergio Lopez Pascual , Stefano Garzarella Subject: Re: [PATCH 7/7] vhost-user: call VHOST_USER_SET_VRING_ENABLE synchronously Message-ID: <20231003104045-mutt-send-email-mst@kernel.org> References: <20230827182937.146450-1-lersek@redhat.com> <20230827182937.146450-8-lersek@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20230827182937.146450-8-lersek@redhat.com> Received-SPF: pass client-ip=170.10.129.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -16 X-Spam_score: -1.7 X-Spam_bar: - X-Spam_report: (-1.7 / 5.0 requ) BAYES_00=-1.9, DKIM_INVALID=0.1, DKIM_SIGNED=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=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: 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 Sun, Aug 27, 2023 at 08:29:37PM +0200, Laszlo Ersek wrote: > (1) The virtio-1.0 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’s virtio configuration space, and population of virtqueues. > > > > 8. Set the DRIVER_OK status bit. At this point the device is “live”. > > 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. > > > > [...] > > 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) is a *control plane* > operation, which 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 steps (1.1) 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 (1.1) -- i.e., in the write to the "queue_enable" register -- 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 So you want me to hold on to this patch 7/7 for now? And maybe merge rest of the patchset? > --- > hw/virtio/vhost-user.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c > index beb4b832245e..01e0ca90c538 100644 > --- a/hw/virtio/vhost-user.c > +++ b/hw/virtio/vhost-user.c > @@ -1235,7 +1235,7 @@ static int vhost_user_set_vring_enable(struct vhost_dev *dev, int enable) > .num = enable, > }; > > - ret = vhost_set_vring(dev, VHOST_USER_SET_VRING_ENABLE, &state, false); > + ret = vhost_set_vring(dev, VHOST_USER_SET_VRING_ENABLE, &state, true); > if (ret < 0) { > /* > * Restoring the previous state is likely infeasible, as well as