[PATCH v3 12/19] vhost: add vhost_svq_poll

["Eugenio Pérez" <eperezma@redhat.com>]

It allows the Shadow Control VirtQueue to wait for the device to use the
available buffers.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
 hw/virtio/vhost-shadow-virtqueue.h |  1 +
 hw/virtio/vhost-shadow-virtqueue.c | 22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/hw/virtio/vhost-shadow-virtqueue.h b/hw/virtio/vhost-shadow-virtqueue.h
index dd78f4bec2..cf442f7dea 100644
--- a/hw/virtio/vhost-shadow-virtqueue.h
+++ b/hw/virtio/vhost-shadow-virtqueue.h
@@ -89,6 +89,7 @@ void vhost_svq_push_elem(VhostShadowVirtqueue *svq,
 int vhost_svq_add(VhostShadowVirtqueue *svq, const struct iovec *out_sg,
                   size_t out_num, const struct iovec *in_sg, size_t in_num,
                   VirtQueueElement *elem);
+size_t vhost_svq_poll(VhostShadowVirtqueue *svq);
 
 void vhost_svq_set_svq_kick_fd(VhostShadowVirtqueue *svq, int svq_kick_fd);
 void vhost_svq_set_svq_call_fd(VhostShadowVirtqueue *svq, int call_fd);
diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
index 406a823c81..1c54a03e17 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c
@@ -484,6 +484,28 @@ static void vhost_svq_flush(VhostShadowVirtqueue *svq,
     } while (!vhost_svq_enable_notification(svq));
 }
 
+/**
+ * Poll the SVQ for one device used buffer.
+ *
+ * This function race with main event loop SVQ polling, so extra
+ * synchronization is needed.
+ *
+ * Return the length written by the device.
+ */
+size_t vhost_svq_poll(VhostShadowVirtqueue *svq)
+{
+    do {
+        uint32_t len;
+        VirtQueueElement *elem = vhost_svq_get_buf(svq, &len);
+        if (elem) {
+            return len;
+        }
+
+        /* Make sure we read new used_idx */
+        smp_rmb();
+    } while (true);
+}
+
 /**
  * Forward used buffers.
  *
-- 
2.31.1