[PATCH 1/3] virtio: add virtioqueue_more_used()

[Stefan Hajnoczi <stefanha@redhat.com>]

Add an API to check whether there are pending used buffers. There is
already a similar API called virtqueue_poll() but it only works together
with virtqueue_enable_cb_prepare(). The patches that follow add blk-mq
->poll() support to virtio_blk and they need to check for used buffers
without re-enabling virtqueue callbacks, so introduce an API for it.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 include/linux/virtio.h       |  2 ++
 drivers/virtio/virtio_ring.c | 17 +++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index b1894e0323fa..c6ad0f25f412 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -63,6 +63,8 @@ bool virtqueue_kick_prepare(struct virtqueue *vq);
 
 bool virtqueue_notify(struct virtqueue *vq);
 
+bool virtqueue_more_used(const struct virtqueue *vq);
+
 void *virtqueue_get_buf(struct virtqueue *vq, unsigned int *len);
 
 void *virtqueue_get_buf_ctx(struct virtqueue *vq, unsigned int *len,
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 71e16b53e9c1..7c3da75da462 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -2032,6 +2032,23 @@ static inline bool more_used(const struct vring_virtqueue *vq)
 	return vq->packed_ring ? more_used_packed(vq) : more_used_split(vq);
 }
 
+/**
+ * virtqueue_more_used - check if there are used buffers pending
+ * @_vq: the struct virtqueue we're talking about.
+ *
+ * Returns true if there are used buffers, false otherwise. May be called at
+ * the same time as other virtqueue operations, but actually calling
+ * virtqueue_get_buf() requires serialization so be mindful of the race between
+ * calling virtqueue_more_used() and virtqueue_get_buf().
+ */
+bool virtqueue_more_used(const struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+
+	return more_used(vq);
+}
+EXPORT_SYMBOL_GPL(virtqueue_more_used);
+
 irqreturn_t vring_interrupt(int irq, void *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
-- 
2.31.1