On Wed, Oct 04, 2023 at 12:15:48PM +0200, Laszlo Ersek wrote: > On 10/3/23 17:55, Stefan Hajnoczi wrote: > > On Tue, 3 Oct 2023 at 10:41, Michael S. Tsirkin <m...@redhat.com> wrote: > >> > >> On Sun, Aug 27, 2023 at 08:29:37PM +0200, Laszlo Ersek wrote: > >>> (1) The virtio-1.0 specification > >>> <http://docs.oasis-open.org/virtio/virtio/v1.0/virtio-v1.0.html> 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 > >>> <https://qemu-project.gitlab.io/qemu/interop/vhost-user.html#ring-states>. > >>> 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" <m...@redhat.com> (supporter:vhost) > >>> Cc: Eugenio Perez Martin <epere...@redhat.com> > >>> Cc: German Maglione <gmagli...@redhat.com> > >>> Cc: Liu Jiang <ge...@linux.alibaba.com> > >>> Cc: Sergio Lopez Pascual <s...@redhat.com> > >>> Cc: Stefano Garzarella <sgarz...@redhat.com> > >>> Signed-off-by: Laszlo Ersek <ler...@redhat.com> > >> > >> > >> So you want me to hold on to this patch 7/7 for now? > >> And maybe merge rest of the patchset? > > > > Up to Laszlo, but I wanted to mention that I support merging this > > patch series. A ring has not been enabled/disabled until the back-end > > replies, so I think this patch series makes sense. > > Sorry, I didn't get to see this part of the discussion yesterday, and > now I see that Michael has gone ahead with a PR that contains v2 of this > set. The night before yesterday I posted v3 > <https://patchwork.ozlabs.org/project/qemu-devel/cover/20231002203221.17241-1-ler...@redhat.com/>, > with commit message updates / improvements only (based on feedback), so > please merge that one. > > Thanks! > Laszlo
OK. I'll need to do another PR soonish since a bunch of patchsets which I wanted in this PR had issues and I had to drop them. v3 will be there. -- MST
