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 349C0C83F3E for ; Tue, 5 Sep 2023 06:32:01 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qdPaT-0001S2-Gh; Tue, 05 Sep 2023 02:30:57 -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 1qdPaM-0001RA-Nh for qemu-devel@nongnu.org; Tue, 05 Sep 2023 02:30:52 -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 1qdPaI-0001RH-Pf for qemu-devel@nongnu.org; Tue, 05 Sep 2023 02:30:50 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1693895445; 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=NkmogD3gAauft644MrTSJziKRmTuuI5T8pFi4/0M1us=; b=JGH1TKrWjm7tlf5KN4TxjmRWLwRIKy51Mx5A26oKI2ZNEnFwc1SXJl0Ho0HOkdKRuI6YE7 Hc2xfODnqNqqQ7JSjmbFOgb6e5WeSZm/VoMnVpiIoyFkr+i8brWlwU3SPGEeAkGjrz3PLu 8VmE48rHOOo8AHdUDW1y8Y8YUH07OR8= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-146-k2V17qJGMf2Uq3Xqe9oDnQ-1; Tue, 05 Sep 2023 02:30:43 -0400 X-MC-Unique: k2V17qJGMf2Uq3Xqe9oDnQ-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 93A193820F08; Tue, 5 Sep 2023 06:30:43 +0000 (UTC) Received: from [10.39.192.226] (unknown [10.39.192.226]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D5A172026D4B; Tue, 5 Sep 2023 06:30:41 +0000 (UTC) Message-ID: <90e566a6-2ac9-5adf-233c-085df81526c0@redhat.com> Date: Tue, 5 Sep 2023 08:30:40 +0200 MIME-Version: 1.0 Subject: Re: [PATCH 7/7] vhost-user: call VHOST_USER_SET_VRING_ENABLE synchronously Content-Language: en-US To: Stefan Hajnoczi , "Michael S. Tsirkin" Cc: qemu-devel@nongnu.org, Eugenio Perez Martin , German Maglione , Liu Jiang , Sergio Lopez Pascual , Stefano Garzarella References: <20230827182937.146450-1-lersek@redhat.com> <20230827182937.146450-8-lersek@redhat.com> <6d766ab4-b6b8-b64b-1f9d-60c558b56509@redhat.com> From: Laszlo Ersek In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.4 Received-SPF: pass client-ip=170.10.133.124; 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+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Michael, On 8/30/23 17:37, 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? Can you comment please? Thanks Laszlo >>> 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 >>> >> >