[PATCH 1/2] virtio: allow to detach a buffer from the virtqueue

[Greg Kurz <groug@kaod.org>]

It is possible for a device to stop using buffers without pushing them
back to the driver. This is the case for example with the 9p virtio
device: if the driver flushes an in-flight request, the 9p specification
specification [*] mandates the server to "to purge the pending response".
The reply to the flush request indicates that the 9p server has stopped
using the buffers of the flushed in-flight request. But since the server
doesn't push back the associated buffers, they don't go back to the free
list. This leads the virtqueue to end up with a single slot to handle all
the dialog with the device, ie, serialize all I/Os.

This patch hence gives the possibility for device specific code to
explicitly detach a given buffer from the used ring and put it back
to the free list.

[*] http://man.cat-v.org/plan_9/5/flush

Signed-off-by: Greg Kurz <groug@kaod.org>
---
 drivers/virtio/virtio_ring.c |   28 ++++++++++++++++++++++++++++
 include/linux/virtio.h       |    1 +
 2 files changed, 29 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index eb30f3e09a47..886e9d054de3 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -936,6 +936,34 @@ void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
 }
 EXPORT_SYMBOL_GPL(virtqueue_detach_unused_buf);
 
+/**
+ * virtqueue_detach_buf - detach specific buffer
+ * @vq: the struct virtqueue we're talking about.
+ *
+ * Returns NULL or the "data" token handed to virtqueue_add_*().
+ * This should only be used if the driver really knows the buffer
+ * isn't needed anymore by the device.
+ */
+void *virtqueue_detach_buf(struct virtqueue *_vq, void *buf)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	unsigned int i;
+
+	START_USE(vq);
+
+	for (i = 0; i < vq->vring.num; i++) {
+		if (vq->desc_state[i].data != buf)
+			continue;
+		detach_buf(vq, i, NULL);
+		END_USE(vq);
+		return buf;
+	}
+
+	END_USE(vq);
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(virtqueue_detach_buf);
+
 irqreturn_t vring_interrupt(int irq, void *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index 988c7355bc22..850158518ce5 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -80,6 +80,7 @@ bool virtqueue_poll(struct virtqueue *vq, unsigned);
 bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
 
 void *virtqueue_detach_unused_buf(struct virtqueue *vq);
+void *virtqueue_detach_buf(struct virtqueue *vq, void *buf);
 
 unsigned int virtqueue_get_vring_size(struct virtqueue *vq);