virtio: Add ability to detach unused buffers from vrings

There's currently no way for a virtio driver to ask for unused
buffers, so it has to keep a list itself to reclaim them at shutdown.
This is redundant, since virtio_ring stores that information.  So
add a new hook to do this.

Signed-off-by: Shirley Ma <xma@us.ibm.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

2 files changed, 29 insertions, 0 deletions

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 827f7e042610..782b7292a3d8 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -351,6 +351,30 @@ static bool vring_enable_cb(struct virtqueue *_vq)
         return true;
 }
 
+static void *vring_detach_unused_buf(struct virtqueue *_vq)
+{
+        struct vring_virtqueue *vq = to_vvq(_vq);
+        unsigned int i;
+        void *buf;
+
+        START_USE(vq);
+
+        for (i = 0; i < vq->vring.num; i++) {
+                if (!vq->data[i])
+                        continue;
+                /* detach_buf clears data, so grab it now. */
+                buf = vq->data[i];
+                detach_buf(vq, i);
+                END_USE(vq);
+                return buf;
+        }
+        /* That should have freed everything. */
+        BUG_ON(vq->num_free != vq->vring.num);
+
+        END_USE(vq);
+        return NULL;
+}
+
 irqreturn_t vring_interrupt(int irq, void *_vq)
 {
         struct vring_virtqueue *vq = to_vvq(_vq);
@@ -377,6 +401,7 @@ static struct virtqueue_ops vring_vq_ops = {
         .kick = vring_kick,
         .disable_cb = vring_disable_cb,
         .enable_cb = vring_enable_cb,
+        .detach_unused_buf = vring_detach_unused_buf,
 };
 
 struct virtqueue *vring_new_virtqueue(unsigned int num,
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index 057a2e010758..f508c651e53d 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -51,6 +51,9 @@ struct virtqueue {
  *      This re-enables callbacks; it returns "false" if there are pending
  *      buffers in the queue, to detect a possible race between the driver
  *      checking for more work, and enabling callbacks.
+ * @detach_unused_buf: detach first unused buffer
+ *      vq: the struct virtqueue we're talking about.
+ *      Returns NULL or the "data" token handed to add_buf
  *
  * Locking rules are straightforward: the driver is responsible for
  * locking.  No two operations may be invoked simultaneously, with the exception
@@ -71,6 +74,7 @@ struct virtqueue_ops {
 
         void (*disable_cb)(struct virtqueue *vq);
         bool (*enable_cb)(struct virtqueue *vq);
+        void *(*detach_unused_buf)(struct virtqueue *vq);
 };
 
 /**