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Diffstat (limited to 'hw/virtio/vhost-user.c')
| -rw-r--r-- | hw/virtio/vhost-user.c | 16 |
1 files changed, 15 insertions, 1 deletions
diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c index 7e452849fa..427ee0ebfb 100644 --- a/hw/virtio/vhost-user.c +++ b/hw/virtio/vhost-user.c @@ -1225,7 +1225,21 @@ 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 |