| Message ID | 89f5bc9dc88a401b472508586752d8906b7505d6.1335186822.git.mst@redhat.com |
|---|---|
| State | New |
| Headers | show |
Il 23/04/2012 15:19, Michael S. Tsirkin ha scritto: > @@ -694,7 +698,7 @@ static bool vring_notify(VirtIODevice *vdev, VirtQueue *vq) > uint16_t old, new; > bool v; > /* We need to expose used array entries before checking used event. */ > - mb(); > + smp_mb(); This mb() is not in upstream QEMU, only in qemu-kvm. Paolo
On Tue, Apr 24, 2012 at 04:03:16PM +0200, Paolo Bonzini wrote: > Il 23/04/2012 15:19, Michael S. Tsirkin ha scritto: > > @@ -694,7 +698,7 @@ static bool vring_notify(VirtIODevice *vdev, VirtQueue *vq) > > uint16_t old, new; > > bool v; > > /* We need to expose used array entries before checking used event. */ > > - mb(); > > + smp_mb(); > > This mb() is not in upstream QEMU, only in qemu-kvm. > > Paolo Actually patch 1/3 adds this by mistake. I tested all 3 together so missed this but it would break bisect. Will fix, thanks.
diff --git a/hw/virtio.c b/hw/virtio.c index 6449746..def0bf1 100644 --- a/hw/virtio.c +++ b/hw/virtio.c @@ -209,6 +209,10 @@ void virtio_queue_set_notification(VirtQueue *vq, int enable) } else { vring_used_flags_set_bit(vq, VRING_USED_F_NO_NOTIFY); } + if (enable) { + /* Expose avail event/used flags before caller checks the avail idx. */ + smp_mb(); + } } int virtio_queue_ready(VirtQueue *vq) @@ -694,7 +698,7 @@ static bool vring_notify(VirtIODevice *vdev, VirtQueue *vq) uint16_t old, new; bool v; /* We need to expose used array entries before checking used event. */ - mb(); + smp_mb(); /* Always notify when queue is empty (when feature acknowledge) */ if (((vdev->guest_features & (1 << VIRTIO_F_NOTIFY_ON_EMPTY)) && !vq->inuse && vring_avail_idx(vq) == vq->last_avail_idx)) {
This fixes an issue dual to the one fixed by patch 'virtio: add missing mb() on notification' and applies on top. In this case, to enable vq kick to exit to host, qemu writes out used flag then reads the avail index. if these are reordered we get a race: host avail index read: ring is empty guest avail index write guest flag read: exit disabled host used flag write: enable exit which results in a lost exit: host will never be notified about the avail index update. Again, happens in the field but only seems to trigger on some specific hardware. Insert an smp_mb barrier operation to ensure the correct ordering. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> --- hw/virtio.c | 6 +++++- 1 files changed, 5 insertions(+), 1 deletions(-)