Not this actually - v2 of this.
On Sun, Oct 01, 2023 at 03:24:59PM -0400, Michael S. Tsirkin wrote:
> 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
> > >>>>
> > >>>
> > >>
> > >
