On 10/4/23 10:44, Michael S. Tsirkin wrote: > From: Laszlo Ersek <ler...@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" <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> > Reviewed-by: Stefano Garzarella <sgarz...@redhat.com> > Message-Id: <20230830134055.106812-8-ler...@redhat.com> > Reviewed-by: Michael S. Tsirkin <m...@redhat.com> > Signed-off-by: Michael S. Tsirkin <m...@redhat.com> > Reviewed-by: Eugenio Pérez <epere...@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 <https://patchwork.ozlabs.org/project/qemu-devel/cover/20231002203221.17241-1-ler...@redhat.com/>. Thanks, Laszlo
