[RFC PATCH 18/27] vhost: add vhost_vring_poll_rcu

[Eugenio Pérez <eperezma@xxxxxxxxxx>]

Signed-off-by: Eugenio Pérez <eperezma@xxxxxxxxxx>
---
 hw/virtio/vhost-sw-lm-ring.h |  2 ++
 hw/virtio/vhost-sw-lm-ring.c | 18 ++++++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/hw/virtio/vhost-sw-lm-ring.h b/hw/virtio/vhost-sw-lm-ring.h
index c537500d9e..03257d60c1 100644
--- a/hw/virtio/vhost-sw-lm-ring.h
+++ b/hw/virtio/vhost-sw-lm-ring.h
@@ -22,6 +22,8 @@ int vhost_vring_add(VhostShadowVirtqueue *vq, VirtQueueElement *elem);
 
 /* Called within rcu_read_lock().  */
 void vhost_vring_set_notification_rcu(VhostShadowVirtqueue *vq, bool enable);
+/* Called within rcu_read_lock().  */
+bool vhost_vring_poll_rcu(VhostShadowVirtqueue *vq);
 
 void vhost_vring_write_addr(const VhostShadowVirtqueue *vq,
 	                    struct vhost_vring_addr *addr);
diff --git a/hw/virtio/vhost-sw-lm-ring.c b/hw/virtio/vhost-sw-lm-ring.c
index c3244c550e..3652889d8e 100644
--- a/hw/virtio/vhost-sw-lm-ring.c
+++ b/hw/virtio/vhost-sw-lm-ring.c
@@ -34,6 +34,12 @@ typedef struct VhostShadowVirtqueue {
     /* Next free descriptor */
     uint16_t free_head;
 
+    /* Last seen used idx */
+    uint16_t shadow_used_idx;
+
+    /* Next head to consume from device */
+    uint16_t used_idx;
+
     /* Cache for exposed notification flag */
     bool notification;
 
@@ -83,6 +89,18 @@ void vhost_vring_set_notification_rcu(VhostShadowVirtqueue *vq, bool enable)
     smp_mb();
 }
 
+bool vhost_vring_poll_rcu(VhostShadowVirtqueue *vq)
+{
+    if (vq->used_idx != vq->shadow_used_idx) {
+        return true;
+    }
+
+    smp_rmb();
+    vq->shadow_used_idx = virtio_tswap16(vq->vdev, vq->vring.used->idx);
+
+    return vq->used_idx != vq->shadow_used_idx;
+}
+
 static void vhost_vring_write_descs(VhostShadowVirtqueue *vq,
                                     const struct iovec *iovec,
                                     size_t num, bool more_descs, bool write)
-- 
2.18.4