Re: [PULL 30/63] vhost-user: call VHOST_USER_SET_VRING_ENABLE synchronously

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Oct 04, 2023 at 12:11:44PM +0200, Laszlo Ersek wrote:
> On 10/4/23 10:44, Michael S. Tsirkin wrote:
> > From: Laszlo Ersek <lersek@redhat.com>
> > 
> > (1) The virtio-1.2 specification
> > <http://docs.oasis-open.org/virtio/virtio/v1.2/virtio-v1.2.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.
> >>
> >> [...]
> > 
> > (The same statements are present in virtio-1.0 identically, at
> > <http://docs.oasis-open.org/virtio/virtio/v1.0/virtio-v1.0.html>.)
> > 
> > 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" <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>
> > Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
> > Message-Id: <20230830134055.106812-8-lersek@redhat.com>
> > Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > Reviewed-by: Eugenio Pérez <eperezma@redhat.com>
> > ---
> >  hw/virtio/vhost-user.c | 16 +++++++++++++++-
> >  1 file changed, 15 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> > index ae0734d461..eb983ae295 100644
> > --- a/hw/virtio/vhost-user.c
> > +++ b/hw/virtio/vhost-user.c
> > @@ -1214,7 +1214,21 @@ 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);
> > +        /*
> > +         * SET_VRING_ENABLE travels from guest to QEMU to vhost-user backend /
> > +         * control plane thread via unix domain socket. Virtio requests travel
> > +         * from guest to vhost-user backend / data plane thread via eventfd.
> > +         * Even if the guest enables the ring first, and pushes its first virtio
> > +         * request second (conforming to the virtio spec), the data plane thread
> > +         * in the backend may see the virtio request before the control plane
> > +         * thread sees the queue enablement. This causes (in fact, requires) the
> > +         * data plane thread to discard the virtio request (it arrived on a
> > +         * seemingly disabled queue). To prevent this out-of-order delivery,
> > +         * don't let the guest proceed to pushing the virtio request until the
> > +         * backend control plane acknowledges enabling the queue -- IOW, pass
> > +         * wait_for_reply=true below.
> > +         */
> > +        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
> 
> This is not the latest version (v3) of this set -- please see
> <https://patchwork.ozlabs.org/project/qemu-devel/cover/20231002203221.17241-1-lersek@redhat.com/>.
> 
> Thanks,
> Laszlo

Ouch. OK I will drop. Feel free to send v4 tweaking commit message - I
think you wanted to do it anyway right?