vhost: workaround stale vring base

When DPDK app crashes (or quits, or gets killed), a restart of DPDK
app would get stale vring base from QEMU. That would break the kernel
virtio net completely, making it non-work any more, unless a driver
reset is done.

So, instead of getting the stale vring base from QEMU, Huawei suggested
we could get a much saner (and may not the most accurate) vring base
from used->idx. That would work because:

- there is a memory barrier between updating used ring entries and
  used->idx. So, even though we crashed at updating the used ring
  entries, it will not cause any issue, as the guest driver will not
  process those stale used entries, for used-idx is not updated yet.

- DPDK process vring in order, that means a crash may just lead some
  packet retransmission for Tx and drop for Rx.

Suggested-by: Huawei Xie <huawei.xie@intel.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Huawei Xie <huawei.xie@intel.com>

diff --git a/lib/librte_vhost/virtio-net.c b/lib/librte_vhost/virtio-net.c
index 835ab3a..bd7e55e 100644 (file)
--- a/lib/librte_vhost/virtio-net.c
+++ b/lib/librte_vhost/virtio-net.c
@@ -561,6 +561,15 @@ vhost_set_vring_addr(int vid, struct vhost_vring_addr *addr)
                return -1;
        }
 
+       if (vq->last_used_idx != vq->used->idx) {
+               RTE_LOG(WARNING, VHOST_CONFIG,
+                       "last_used_idx (%u) and vq->used->idx (%u) mismatches; "
+                       "some packets maybe resent for Tx and dropped for Rx\n",
+                       vq->last_used_idx, vq->used->idx);
+               vq->last_used_idx     = vq->used->idx;
+               vq->last_used_idx_res = vq->used->idx;
+       }
+
        vq->log_guest_addr = addr->log_guest_addr;
 
        LOG_DEBUG(VHOST_CONFIG, "(%d) mapped address desc: %p\n",