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 5E3D9E7849A for ; Mon, 2 Oct 2023 06:50:19 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qnCke-00053R-TR; Mon, 02 Oct 2023 02:49:56 -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 1qnCka-000539-3R for qemu-devel@nongnu.org; Mon, 02 Oct 2023 02:49:53 -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 1qnCkV-0003BJ-9V for qemu-devel@nongnu.org; Mon, 02 Oct 2023 02:49:49 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1696229385; 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=Mb3fI3Izsi0mJ5jqBRniZJi4yyRXbo328saeebY7r5k=; b=UrOY0payuplZOeVSEmZX9Zf4ON19LlIQuyVDrzytpOUDp7p8WFtdvaCuQcjS0z4tAOV5qg fIGXxWpip7bjsTDfw3rqhBAKGXQYDAYiQ4ZCswvR9e2v/BSGtCPdLuw7a4bA+ag+LkuGL8 3XmJwp11sAQ3+G8GUNhWh8RFaS/t9Ys= Received: from mail-wm1-f69.google.com (mail-wm1-f69.google.com [209.85.128.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-696-R_D8bs5PMpaeMKTcpU2GLA-1; Mon, 02 Oct 2023 02:49:42 -0400 X-MC-Unique: R_D8bs5PMpaeMKTcpU2GLA-1 Received: by mail-wm1-f69.google.com with SMTP id 5b1f17b1804b1-4065478afd3so24838905e9.0 for ; Sun, 01 Oct 2023 23:49:42 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1696229381; x=1696834181; 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=Mb3fI3Izsi0mJ5jqBRniZJi4yyRXbo328saeebY7r5k=; b=d1F+KmHATN2orfkTbiCz0sYkpjwIrXOJLdyG4zrpMPhTxFp7BXoikeaMqO13yu2mk9 A6wMLEB2xgqGzO0++LppmndBsWW1+4ZtNJqPdh3t511uQt0YsSG4+EwRsN0SUat1UcQi al5eSlXndDQOdJfMTEINnb6yTxwKY5WECWFLm9rwZahfNjtlQy42ILs0tM0BHF0iV+q2 +JfOm1yNvciuBuua0niH8eEp2GX45k1+uObnJhWQZWDgK+jBINpxIvcu9bXCLDhF3vr4 sY96eXv17824omMSkOZLqqaFlGMW6XK2dSvT+eB0a0a+LyhisG5cOrpm5SSWHY20p96N yDtA== X-Gm-Message-State: AOJu0Ywah8HxdGqcjQ0DiH+sC98yKTrzrCEdxSHdfcN/J5lIKStHPjoL opkk0nMX1qmd9Q3rsX2nEedZQhXP6bTNNc5Q7ETTvgn+w3PENLaoIOsB7j6uAyTfXiZ+yfgF4UW GmuCuvpbyZinb5Fo= X-Received: by 2002:a05:600c:293:b0:401:1b58:72f7 with SMTP id 19-20020a05600c029300b004011b5872f7mr10042501wmk.38.1696229381170; Sun, 01 Oct 2023 23:49:41 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEeaw7qQm0GZOtrt8Uf3yUM+roBC4M6FjV2Re4hlxlrorRFGHq2PGT0IsF11V+CZ/cp9Mzkag== X-Received: by 2002:a05:600c:293:b0:401:1b58:72f7 with SMTP id 19-20020a05600c029300b004011b5872f7mr10042481wmk.38.1696229380763; Sun, 01 Oct 2023 23:49:40 -0700 (PDT) Received: from redhat.com ([2.52.132.27]) by smtp.gmail.com with ESMTPSA id w14-20020a5d680e000000b00325a59b2080sm6911372wru.97.2023.10.01.23.49.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 01 Oct 2023 23:49:39 -0700 (PDT) Date: Mon, 2 Oct 2023 02:49:36 -0400 From: "Michael S. Tsirkin" To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, Laszlo Ersek , Eugenio Perez Martin , German Maglione , Liu Jiang , Sergio Lopez Pascual , Stefano Garzarella , Jason Wang Subject: Re: [PATCH 7/7] vhost-user: call VHOST_USER_SET_VRING_ENABLE synchronously Message-ID: <20231002015259-mutt-send-email-mst@kernel.org> References: <20230827182937.146450-1-lersek@redhat.com> <20230827182937.146450-8-lersek@redhat.com> <6d766ab4-b6b8-b64b-1f9d-60c558b56509@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Received-SPF: pass client-ip=170.10.133.124; envelope-from=mst@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_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, 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+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Wed, Aug 30, 2023 at 11:37:50AM -0400, Stefan Hajnoczi wrote: > On Wed, 30 Aug 2023 at 09:30, Laszlo Ersek wrote: > > > > On 8/30/23 14:10, Stefan Hajnoczi wrote: > > > On Sun, 27 Aug 2023 at 14:31, 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. > > > > > > Why is virtiofsd monitoring the virtqueue and discarding requests > > > while it's disabled? > > > > That's what the vhost-user spec requires: > > > > https://qemu-project.gitlab.io/qemu/interop/vhost-user.html#ring-states > > > > """ > > started but disabled: the back-end must process the ring without causing > > any side effects. For example, for a networking device, in the disabled > > state the back-end must not supply any new RX packets, but must process > > and discard any TX packets. > > """ > > > > This state is different from "stopped", where "the back-end must not > > process the ring at all". > > > > The spec also says, > > > > """ > > If VHOST_USER_F_PROTOCOL_FEATURES has been negotiated, the ring is > > initialized in a disabled state and is enabled by > > VHOST_USER_SET_VRING_ENABLE with parameter 1. > > """ > > > > AFAICT virtiofsd follows this requirement. > > Hi Michael, > You documented the disabled ring state in QEMU commit commit > c61f09ed855b5009f816242ce281fd01586d4646 ("vhost-user: clarify start > and enable") where virtio-net devices discard tx buffers. The disabled > state seems to be specific to vhost-user and not covered in the VIRTIO > specification. > > Do you remember what the purpose of the disabled state was? Why is it > necessary to discard tx buffers instead of postponing ring processing > until the virtqueue is enabled? > > My concern is that the semantics are unclear for virtqueue types that > are different from virtio-net rx/tx. Even the virtio-net controlq > would be problematic - should buffers be silently discarded with > VIRTIO_NET_OK or should they fail? > > Thanks, > Stefan I think I got it now. This weird state happens when linux first queues packets on multiple queues, then changes max queues to 1, queued packets need to still be freed eventually. Yes, I am not sure this can apply to devices or queue types other than virtio net. Maybe. When we say: must process the ring without causing any side effects. then I think it would be better to say must process the ring if it can be done without causing guest visible side effects. processing rx ring would have a side effect of causing guest to get malformed buffers, so we don't process it. processing command queue - we can't fail for sure since that is guest visible. but practically we don't do this for cvq. what should happen for virtiofsd? I don't know - I am guessing discarding would have a side effect so should not happen. > > > > > This seems like a bug in the vhost-user backend to me. > > > > I didn't want to exclude that possiblity; that's why I included Eugenio, > > German, Liu Jiang, and Sergio in the CC list. > > > > > > > > When the virtqueue is disabled, don't monitor the kickfd. > > > > > > When the virtqueue transitions from disabled to enabled, the control > > > plane should self-trigger the kickfd so that any available buffers > > > will be processed. > > > > > > QEMU uses this scheme to switch between vhost/IOThreads and built-in > > > virtqueue kick processing. > > > > > > This approach is more robust than relying buffers being enqueued after > > > the virtqueue is enabled. > > > > I'm happy to drop the series if the virtiofsd maintainers agree that the > > bug is in virtiofsd, and can propose a design to fix it. (I do think > > that such a fix would require an architectural change.) > > > > FWIW, my own interpretation of the vhost-user spec (see above) was that > > virtiofsd was right to behave the way it did, and that there was simply > > no way to prevent out-of-order delivery other than synchronizing the > > guest end-to-end with the vhost-user backend, concerning > > VHOST_USER_SET_VRING_ENABLE. > > > > This end-to-end synchronization is present "naturally" in vhost-net, > > where ioctl()s are automatically synchronous -- in fact *all* operations > > on the control plane are synchronous. (Which is just a different way to > > say that the guest is tightly coupled with the control plane.) > > > > Note that there has been at least one race like this before; see commit > > 699f2e535d93 ("vhost: make SET_VRING_ADDR, SET_FEATURES send replies", > > 2021-09-04). Basically every pre-existent call to enforce_reply() is a > > cover-up for the vhost-user spec turning (somewhat recklessly?) most > > operations into async ones. > > > > At some point this became apparent and so the REPLY_ACK flag was > > introduced; see commit ca525ce5618b ("vhost-user: Introduce a new > > protocol feature REPLY_ACK.", 2016-08-10). (That commit doesn't go into > > details, but I'm pretty sure there was a similar race around SET_MEM_TABLE!) > > > > BTW even if we drop this series for QEMU, I don't think it will have > > been in vain. The first few patches are cleanups which could be merged > > for their own sake. And the last patch is essentially the proof of the > > problem statement / analysis. It can be considered an elaborate bug > > report for virtiofsd, *if* we decide the bug is in virtiofsd. I did have > > that avenue in mind as well, when writing the commit message / patch. > > > > For now I'm going to post v2 -- that's not to say that I'm dismissing > > your feedback (see above!), just want to get the latest version on-list. > > > > Thanks! > > Laszlo > > > > > > > > Stefan > > > > > >> > > >> 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 > > >> --- > > >> 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 > > > > >