[dpdk-dev] [PATCH v3 4/6] vhost: workaround stale vring base

[Yuanhan Liu]

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.

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

v2: log on packets resent for Tx and drop for Rx
---
 lib/librte_vhost/virtio-net.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/lib/librte_vhost/virtio-net.c b/lib/librte_vhost/virtio-net.c
index 835ab3a..bd7e55e 100644
--- 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",
-- 
1.9.0