[RFC PATCH 17/27] vhost: add vhost_vring_set_notification_rcu

[Eugenio Pérez <eperezma@xxxxxxxxxx>]

Signed-off-by: Eugenio Pérez <eperezma@xxxxxxxxxx>
---
 hw/virtio/vhost-sw-lm-ring.h |  4 ++++
 hw/virtio/vhost-sw-lm-ring.c | 23 +++++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/hw/virtio/vhost-sw-lm-ring.h b/hw/virtio/vhost-sw-lm-ring.h
index 29d21feaf4..c537500d9e 100644
--- a/hw/virtio/vhost-sw-lm-ring.h
+++ b/hw/virtio/vhost-sw-lm-ring.h
@@ -19,6 +19,10 @@ typedef struct VhostShadowVirtqueue VhostShadowVirtqueue;
 
 bool vhost_vring_kick(VhostShadowVirtqueue *vq);
 int vhost_vring_add(VhostShadowVirtqueue *vq, VirtQueueElement *elem);
+
+/* Called within rcu_read_lock().  */
+void vhost_vring_set_notification_rcu(VhostShadowVirtqueue *vq, bool enable);
+
 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 aed005c2d9..c3244c550e 100644
--- a/hw/virtio/vhost-sw-lm-ring.c
+++ b/hw/virtio/vhost-sw-lm-ring.c
@@ -34,6 +34,9 @@ typedef struct VhostShadowVirtqueue {
     /* Next free descriptor */
     uint16_t free_head;
 
+    /* Cache for exposed notification flag */
+    bool notification;
+
     vring_desc_t descs[];
 } VhostShadowVirtqueue;
 
@@ -60,6 +63,26 @@ bool vhost_vring_kick(VhostShadowVirtqueue *vq)
                                        : true;
 }
 
+void vhost_vring_set_notification_rcu(VhostShadowVirtqueue *vq, bool enable)
+{
+    uint16_t notification_flag;
+
+    if (vq->notification == enable) {
+        return;
+    }
+
+    notification_flag = virtio_tswap16(vq->vdev, VRING_AVAIL_F_NO_INTERRUPT);
+
+    vq->notification = enable;
+    if (enable) {
+        vq->vring.avail->flags &= ~notification_flag;
+    } else {
+        vq->vring.avail->flags |= notification_flag;
+    }
+
+    smp_mb();
+}
+
 static void vhost_vring_write_descs(VhostShadowVirtqueue *vq,
                                     const struct iovec *iovec,
                                     size_t num, bool more_descs, bool write)
-- 
2.18.4