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Tsirkin" To: Laszlo Ersek Cc: Stefan Hajnoczi , 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: <20231001152422-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> <90e566a6-2ac9-5adf-233c-085df81526c0@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=170.10.129.124; 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_H4=-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 yes sorry - I am working on a pull request with this included. On Mon, Sep 25, 2023 at 05:31:17PM +0200, Laszlo Ersek wrote: > Ping -- Michael, any comments please? This set (now at v2) has been > waiting on your answer since Aug 30th. > > Laszlo > > On 9/5/23 08:30, Laszlo Ersek wrote: > > 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 > >>>> > >>> > >> > >