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. Stefan
