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

[Laszlo Ersek <lersek@redhat.com>]

Ping -- Michael, any comments please? This set (now at v2) has been
waiting on your answer since Aug 30th.

Laszlo

On 9/5/23 08:30, Laszlo Ersek wrote:
> Michael,
> 
> On 8/30/23 17:37, Stefan Hajnoczi wrote:
>> On Wed, 30 Aug 2023 at 09:30, Laszlo Ersek <lersek@redhat.com> wrote:
>>>
>>> On 8/30/23 14:10, Stefan Hajnoczi wrote:
>>>> 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?
>>>
>>> That's what the vhost-user spec requires:
>>>
>>> https://qemu-project.gitlab.io/qemu/interop/vhost-user.html#ring-states
>>>
>>> """
>>> started but disabled: the back-end must process the ring without causing
>>> any side effects. For example, for a networking device, in the disabled
>>> state the back-end must not supply any new RX packets, but must process
>>> and discard any TX packets.
>>> """
>>>
>>> This state is different from "stopped", where "the back-end must not
>>> process the ring at all".
>>>
>>> The spec also says,
>>>
>>> """
>>> If VHOST_USER_F_PROTOCOL_FEATURES has been negotiated, the ring is
>>> initialized in a disabled state and is enabled by
>>> VHOST_USER_SET_VRING_ENABLE with parameter 1.
>>> """
>>>
>>> AFAICT virtiofsd follows this requirement.
>>
>> Hi Michael,
>> You documented the disabled ring state in QEMU commit commit
>> c61f09ed855b5009f816242ce281fd01586d4646 ("vhost-user: clarify start
>> and enable") where virtio-net devices discard tx buffers. The disabled
>> state seems to be specific to vhost-user and not covered in the VIRTIO
>> specification.
>>
>> Do you remember what the purpose of the disabled state was? Why is it
>> necessary to discard tx buffers instead of postponing ring processing
>> until the virtqueue is enabled?
>>
>> My concern is that the semantics are unclear for virtqueue types that
>> are different from virtio-net rx/tx. Even the virtio-net controlq
>> would be problematic - should buffers be silently discarded with
>> VIRTIO_NET_OK or should they fail?
> 
> Can you comment please?
> 
> Thanks
> Laszlo
> 
> 
>>>> This seems like a bug in the vhost-user backend to me.
>>>
>>> I didn't want to exclude that possiblity; that's why I included Eugenio,
>>> German, Liu Jiang, and Sergio in the CC list.
>>>
>>>>
>>>> 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.
>>>
>>> I'm happy to drop the series if the virtiofsd maintainers agree that the
>>> bug is in virtiofsd, and can propose a design to fix it. (I do think
>>> that such a fix would require an architectural change.)
>>>
>>> FWIW, my own interpretation of the vhost-user spec (see above) was that
>>> virtiofsd was right to behave the way it did, and that there was simply
>>> no way to prevent out-of-order delivery other than synchronizing the
>>> guest end-to-end with the vhost-user backend, concerning
>>> VHOST_USER_SET_VRING_ENABLE.
>>>
>>> This end-to-end synchronization is present "naturally" in vhost-net,
>>> where ioctl()s are automatically synchronous -- in fact *all* operations
>>> on the control plane are synchronous. (Which is just a different way to
>>> say that the guest is tightly coupled with the control plane.)
>>>
>>> Note that there has been at least one race like this before; see commit
>>> 699f2e535d93 ("vhost: make SET_VRING_ADDR, SET_FEATURES send replies",
>>> 2021-09-04). Basically every pre-existent call to enforce_reply() is a
>>> cover-up for the vhost-user spec turning (somewhat recklessly?) most
>>> operations into async ones.
>>>
>>> At some point this became apparent and so the REPLY_ACK flag was
>>> introduced; see commit ca525ce5618b ("vhost-user: Introduce a new
>>> protocol feature REPLY_ACK.", 2016-08-10). (That commit doesn't go into
>>> details, but I'm pretty sure there was a similar race around SET_MEM_TABLE!)
>>>
>>> BTW even if we drop this series for QEMU, I don't think it will have
>>> been in vain. The first few patches are cleanups which could be merged
>>> for their own sake. And the last patch is essentially the proof of the
>>> problem statement / analysis. It can be considered an elaborate bug
>>> report for virtiofsd, *if* we decide the bug is in virtiofsd. I did have
>>> that avenue in mind as well, when writing the commit message / patch.
>>>
>>> For now I'm going to post v2 -- that's not to say that I'm dismissing
>>> your feedback (see above!), just want to get the latest version on-list.
>>>
>>> Thanks!
>>> Laszlo
>>>
>>>>
>>>> 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
>>>>
>>>
>>
>