yes sorry - I am working on a pull request with this
included.
On Mon, Sep 25, 2023 at 05:31:17PM +0200, Laszlo Ersek wrote:
> 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 <ler...@redhat.com> wrote:
> >>>
> >>> On 8/30/23 14:10, Stefan Hajnoczi wrote:
> >>>> On Sun, 27 Aug 2023 at 14:31, Laszlo Ersek <ler...@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" <m...@redhat.com> (supporter:vhost)
> >>>>> Cc: Eugenio Perez Martin <epere...@redhat.com>
> >>>>> Cc: German Maglione <gmagli...@redhat.com>
> >>>>> Cc: Liu Jiang <ge...@linux.alibaba.com>
> >>>>> Cc: Sergio Lopez Pascual <s...@redhat.com>
> >>>>> Cc: Stefano Garzarella <sgarz...@redhat.com>
> >>>>> Signed-off-by: Laszlo Ersek <ler...@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
> >>>>
> >>>
> >>
> >
