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From: Laszlo Ersek <lersek@redhat.com>
To: qemu-devel@nongnu.org, lersek@redhat.com
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	Eugenio Perez Martin <eperezma@redhat.com>,
	German Maglione <gmaglione@redhat.com>,
	Liu Jiang <gerry@linux.alibaba.com>,
	Sergio Lopez Pascual <slp@redhat.com>,
	Stefano Garzarella <sgarzare@redhat.com>
Subject: [PATCH 7/7] vhost-user: call VHOST_USER_SET_VRING_ENABLE synchronously
Date: Sun, 27 Aug 2023 20:29:37 +0200	[thread overview]
Message-ID: <20230827182937.146450-8-lersek@redhat.com> (raw)
In-Reply-To: <20230827182937.146450-1-lersek@redhat.com>

(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" <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

  parent reply	other threads:[~2023-08-27 18:30 UTC|newest]

Thread overview: 58+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-08-27 18:29 [PATCH 0/7] vhost-user: call VHOST_USER_SET_VRING_ENABLE synchronously Laszlo Ersek
2023-08-27 18:29 ` [PATCH 1/7] vhost-user: strip superfluous whitespace Laszlo Ersek
2023-08-30  8:26   ` Stefano Garzarella
2023-08-30 15:04   ` Philippe Mathieu-Daudé
2023-08-27 18:29 ` [PATCH 2/7] vhost-user: tighten "reply_supported" scope in "set_vring_addr" Laszlo Ersek
2023-08-30  8:27   ` Stefano Garzarella
2023-08-30 15:04   ` Philippe Mathieu-Daudé
2023-08-27 18:29 ` [PATCH 3/7] vhost-user: factor out "vhost_user_write_msg" Laszlo Ersek
2023-08-28 22:46   ` Philippe Mathieu-Daudé
2023-08-30  8:31   ` Stefano Garzarella
2023-08-30  9:14     ` Laszlo Ersek
2023-08-30  9:54       ` Laszlo Ersek
2023-08-27 18:29 ` [PATCH 4/7] vhost-user: flatten "enforce_reply" into "vhost_user_write_msg" Laszlo Ersek
2023-08-28 22:47   ` Philippe Mathieu-Daudé
2023-08-30  8:31   ` Stefano Garzarella
2023-08-27 18:29 ` [PATCH 5/7] vhost-user: hoist "write_msg", "get_features", "get_u64" Laszlo Ersek
2023-08-30  8:32   ` Stefano Garzarella
2023-08-30 15:04   ` Philippe Mathieu-Daudé
2023-08-27 18:29 ` [PATCH 6/7] vhost-user: allow "vhost_set_vring" to wait for a reply Laszlo Ersek
2023-08-28 22:49   ` Philippe Mathieu-Daudé
2023-08-30  8:32   ` Stefano Garzarella
2023-08-27 18:29 ` Laszlo Ersek [this message]
2023-08-30  8:39   ` [PATCH 7/7] vhost-user: call VHOST_USER_SET_VRING_ENABLE synchronously Stefano Garzarella
2023-08-30  9:26     ` Laszlo Ersek
2023-08-30 14:24       ` Stefano Garzarella
2023-08-30  8:41   ` Laszlo Ersek
2023-08-30  8:59     ` Laszlo Ersek
2023-08-30  9:04       ` Laszlo Ersek
2023-08-30 12:10   ` Stefan Hajnoczi
2023-08-30 13:30     ` Laszlo Ersek
2023-08-30 15:37       ` Stefan Hajnoczi
2023-09-05  6:30         ` Laszlo Ersek
2023-09-25 15:31           ` Laszlo Ersek
2023-10-01 19:24             ` Michael S. Tsirkin
2023-10-01 19:25               ` Michael S. Tsirkin
2023-10-02  1:56                 ` Laszlo Ersek
2023-10-02  6:57                   ` Michael S. Tsirkin
2023-10-02 14:02                     ` Laszlo Ersek
2023-10-02  6:49         ` Michael S. Tsirkin
2023-10-02 21:12           ` Stefan Hajnoczi
2023-10-02 21:13             ` Stefan Hajnoczi
2023-10-03 12:26               ` Michael S. Tsirkin
2023-10-03 13:08                 ` Stefan Hajnoczi
2023-10-03 13:23                   ` Laszlo Ersek
2023-10-03 14:25                     ` Michael S. Tsirkin
2023-10-03 14:28                       ` Laszlo Ersek
2023-10-03 14:40                   ` Michael S. Tsirkin
2023-10-03 15:45                     ` Stefan Hajnoczi
2023-10-02 22:36             ` Michael S. Tsirkin
2023-10-03  0:17               ` Stefan Hajnoczi
2023-10-03 14:28                 ` Michael S. Tsirkin
2023-10-03 14:41   ` Michael S. Tsirkin
2023-10-03 15:55     ` Stefan Hajnoczi
2023-10-04 10:15       ` Laszlo Ersek
2023-10-04 16:30         ` Michael S. Tsirkin
2023-10-04 10:17       ` Laszlo Ersek
2023-08-30  8:48 ` [PATCH 0/7] " Stefano Garzarella
2023-08-30  9:32   ` Laszlo Ersek

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