virtio-scsi: Report missed events

When an event is reported but no buffers are present in the event vq,
we can set a flag and report a dummy event as soon as one is added.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/virtio-scsi.c

@@ -141,6 +141,7 @@ typedef struct {
     uint32_t sense_size;
     uint32_t cdb_size;
     int resetting;
+    bool events_dropped;
     VirtQueue *ctrl_vq;
     VirtQueue *event_vq;
     VirtQueue *cmd_vqs[0];
@@ -416,10 +417,6 @@ static void virtio_scsi_handle_ctrl(VirtIODevice *vdev, VirtQueue *vq)
     }
 }
 
-static void virtio_scsi_handle_event(VirtIODevice *vdev, VirtQueue *vq)
-{
-}
-
 static void virtio_scsi_command_complete(SCSIRequest *r, uint32_t status,
                                          size_t resid)
 {
@@ -594,22 +591,34 @@ static void virtio_scsi_push_event(VirtIOSCSI *s, SCSIDevice *dev,
 {
     VirtIOSCSIReq *req = virtio_scsi_pop_req(s, s->event_vq);
     VirtIOSCSIEvent *evt;
+    int in_size;
 
-    if (req) {
-        int in_size;
-        if (req->elem.out_num || req->elem.in_num != 1) {
-            virtio_scsi_bad_req();
-        }
+    if (!req) {
+        s->events_dropped = true;
+        return;
+    }
 
-        in_size = req->elem.in_sg[0].iov_len;
-        if (in_size < sizeof(VirtIOSCSIEvent)) {
-            virtio_scsi_bad_req();
-        }
+    if (req->elem.out_num || req->elem.in_num != 1) {
+        virtio_scsi_bad_req();
+    }
 
-        evt = req->resp.event;
-        memset(evt, 0, sizeof(VirtIOSCSIEvent));
-        evt->event = event;
-        evt->reason = reason;
+    if (s->events_dropped) {
+        event |= VIRTIO_SCSI_T_EVENTS_MISSED;
+        s->events_dropped = false;
+    }
+
+    in_size = req->elem.in_sg[0].iov_len;
+    if (in_size < sizeof(VirtIOSCSIEvent)) {
+        virtio_scsi_bad_req();
+    }
+
+    evt = req->resp.event;
+    memset(evt, 0, sizeof(VirtIOSCSIEvent));
+    evt->event = event;
+    evt->reason = reason;
+    if (!dev) {
+        assert(event == VIRTIO_SCSI_T_NO_EVENT);
+    } else {
         evt->lun[0] = 1;
         evt->lun[1] = dev->id;
 
@@ -618,7 +627,16 @@ static void virtio_scsi_push_event(VirtIOSCSI *s, SCSIDevice *dev,
             evt->lun[2] = (dev->lun >> 8) | 0x40;
         }
         evt->lun[3] = dev->lun & 0xFF;
-        virtio_scsi_complete_req(req);
+    }
+    virtio_scsi_complete_req(req);
+}
+
+static void virtio_scsi_handle_event(VirtIODevice *vdev, VirtQueue *vq)
+{
+    VirtIOSCSI *s = (VirtIOSCSI *)vdev;
+
+    if (s->events_dropped) {
+        virtio_scsi_push_event(s, NULL, VIRTIO_SCSI_T_NO_EVENT, 0);
     }
 }