libvhost-user: support resuming vq->last_avail_idx based on used_idx

This is the same workaround as commit 523b018d, which was lost
with libvhost-user transition in commit e10e798c.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 35480cbf)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

contrib/libvhost-user/libvhost-user.c

@@ -521,6 +521,19 @@ vu_set_vring_addr_exec(VuDev *dev, VhostUserMsg *vmsg)
 
     vq->used_idx = vq->vring.used->idx;
 
+    if (vq->last_avail_idx != vq->used_idx) {
+        bool resume = dev->iface->queue_is_processed_in_order &&
+            dev->iface->queue_is_processed_in_order(dev, index);
+
+        DPRINT("Last avail index != used index: %u != %u%s\n",
+               vq->last_avail_idx, vq->used_idx,
+               resume ? ", resuming" : "");
+
+        if (resume) {
+            vq->shadow_avail_idx = vq->last_avail_idx = vq->used_idx;
+        }
+    }
+
     return false;
 }
 

contrib/libvhost-user/libvhost-user.h

@@ -132,6 +132,7 @@ typedef void (*vu_set_features_cb) (VuDev *dev, uint64_t features);
 typedef int (*vu_process_msg_cb) (VuDev *dev, VhostUserMsg *vmsg,
                                   int *do_reply);
 typedef void (*vu_queue_set_started_cb) (VuDev *dev, int qidx, bool started);
+typedef bool (*vu_queue_is_processed_in_order_cb) (VuDev *dev, int qidx);
 
 typedef struct VuDevIface {
     /* called by VHOST_USER_GET_FEATURES to get the features bitmask */
@@ -148,6 +149,12 @@ typedef struct VuDevIface {
     vu_process_msg_cb process_msg;
     /* tells when queues can be processed */
     vu_queue_set_started_cb queue_set_started;
+    /*
+     * If the queue is processed in order, in which case it will be
+     * resumed to vring.used->idx. This can help to support resuming
+     * on unmanaged exit/crash.
+     */
+    vu_queue_is_processed_in_order_cb queue_is_processed_in_order;
 } VuDevIface;
 
 typedef void (*vu_queue_handler_cb) (VuDev *dev, int qidx);