Re: [PATCH 7/7] vhost-user: call VHOST_USER_SET_VRING_ENABLE synchronously

[Stefan Hajnoczi]

On Sun, 27 Aug 2023 at 14:31, Laszlo Ersek <lersek@redhat.com> 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.

Why is virtiofsd monitoring the virtqueue and discarding requests
while it's disabled? This seems like a bug in the vhost-user backend
to me.

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.

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" <mst@redhat.com> (supporter:vhost)
> Cc: Eugenio Perez Martin <eperezma@redhat.com>
> Cc: German Maglione <gmaglione@redhat.com>
> Cc: Liu Jiang <gerry@linux.alibaba.com>
> Cc: Sergio Lopez Pascual <slp@redhat.com>
> Cc: Stefano Garzarella <sgarzare@redhat.com>
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>
> ---
>  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