vhost: fix virtio freeze due to missed interrupt

Update of used->idx and read of avail->flags could be reordered.
Memory fence should be used to ensure the order, otherwise guest could see
a stale used->idx value after it toggles the interrupt suppression flag.
After guest sets the interrupt suppression flag, it will check if there
is more buffer to process through used->idx. If it sees a stale value,
it will exit the processing while host won't send interrupt to guest.

Signed-off-by: Huawei Xie <huawei.xie@intel.com>
Reviewed-by: Luke Gorrie <luke@snabb.co>

diff --git a/lib/librte_vhost/vhost_rxtx.c b/lib/librte_vhost/vhost_rxtx.c
index 510ffe8..4809d32 100644 (file)
--- a/lib/librte_vhost/vhost_rxtx.c
+++ b/lib/librte_vhost/vhost_rxtx.c
@@ -178,6 +178,9 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t queue_id,
        *(volatile uint16_t *)&vq->used->idx += count;
        vq->last_used_idx = res_end_idx;
 
+       /* flush used->idx update before we read avail->flags. */
+       rte_mb();
+
        /* Kick the guest if necessary. */
        if (!(vq->avail->flags & VRING_AVAIL_F_NO_INTERRUPT))
                eventfd_write((int)vq->callfd, 1);
@@ -505,6 +508,9 @@ virtio_dev_merge_rx(struct virtio_net *dev, uint16_t queue_id,
                *(volatile uint16_t *)&vq->used->idx += entry_success;
                vq->last_used_idx = res_end_idx;
 
+               /* flush used->idx update before we read avail->flags. */
+               rte_mb();
+
                /* Kick the guest if necessary. */
                if (!(vq->avail->flags & VRING_AVAIL_F_NO_INTERRUPT))
                        eventfd_write((int)vq->callfd, 1);