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 >>>> >>> >> >
