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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: <20231002183410-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> <20231002015259-mutt-send-email-mst@kernel.org> 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=0.001, 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 On Mon, Oct 02, 2023 at 05:12:27PM -0400, Stefan Hajnoczi wrote: > On Mon, 2 Oct 2023 at 02:49, Michael S. Tsirkin wrote: > > > > 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. > > Can you explain what is happening in the guest driver, QEMU, and the > vhost-user-net device in more detail? I don't understand the scenario. guest changes max vq pairs making it smaller qemu disables ring > > 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. > > Completing a tx buffer is guest-visible, so I'm confused by this statement. yes but it's not immediately guest visible whether packet was transmitted or discarded. > > processing rx ring would have a side effect of causing > > guest to get malformed buffers, so we don't process it. > > Why are they malformed? Do you mean the rx buffers are stale (the > guest driver has changed the number of queues and doesn't expect to > receive them anymore)? there's no way to consume an rx buffer without supplying an rx packet to guest. > > 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 > > > > > > > > > > >