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

[Laszlo Ersek]

On 10/4/23 14:53, Michael S. Tsirkin wrote:
> 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
>> <20231002203221.17241-1-lersek@redhat.com/">https://patchwork.ozlabs.org/project/qemu-devel/cover/20231002203221.17241-1-lersek@redhat.com/>.
>>
>> Thanks,
>> Laszlo
> 
> Ouch. OK I will drop.

Thanks!

> Feel free to send v4 tweaking commit message - I
> think you wanted to do it anyway right?

The v3 series (already on list) is the latest / most recent version, and
that one already includes the intended commit message tweaks. So there's
no need for me to post another version (i.e., no need for a v4); just
please replace my patches in this PR with the v3 series.

Thanks!
Laszlo