[PATCHv2-RFC,1/2] virtio: support layout with avail ring before idx

Commit Message

Michael S. Tsirkin May 26, 2010, 7:50 p.m. UTC

None

Patch

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 1ca8890..2241342 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -224,7 +224,7 @@  add_head:
 	/* Put entry in available array (but don't update avail->idx until they
 	 * do sync).  FIXME: avoid modulus here? */
 	avail = (vq->vring.avail->idx + vq->num_added++) % vq->vring.num;
-	vq->vring.avail->ring[avail] = head;
+	vq->vring.avail_ring[avail] = head;
 
 	pr_debug("Added buffer head %i to %p\n", head, vq);
 	END_USE(vq);
@@ -425,7 +425,7 @@  struct virtqueue *vring_new_virtqueue(unsigned int num,
 	if (!vq)
 		return NULL;
 
-	vring_init(&vq->vring, num, pages, vring_align);
+	vring_init(&vq->vring, num, pages, vring_align, false);
 	vq->vq.callback = callback;
 	vq->vq.vdev = vdev;
 	vq->vq.name = name;
diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index e4d144b..c5f3ee7 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -47,6 +47,11 @@  struct vring_avail {
 	__u16 ring[];
 };
 
+struct vring_avail_ctrl {
+	__u16 flags;
+	__u16 idx;
+};
+
 /* u32 is used here for ids for padding reasons. */
 struct vring_used_elem {
 	/* Index of start of used descriptor chain. */
@@ -66,7 +71,9 @@  struct vring {
 
 	struct vring_desc *desc;
 
-	struct vring_avail *avail;
+	struct vring_avail_ctrl *avail;
+
+	__u16 *avail_ring;
 
 	struct vring_used *used;
 };
@@ -79,11 +86,18 @@  struct vring {
  *	// The actual descriptors (16 bytes each)
  *	struct vring_desc desc[num];
  *
- *	// A ring of available descriptor heads with free-running index.
+ *	// A ring of available descriptor heads with a control structure
+ *      // including a free-running index.
+ *      // The ring can come either after  (legacy) or before the control.
  *	__u16 avail_flags;
  *	__u16 avail_idx;
  *	__u16 available[num];
  *
+ * or
+ *
+ *	__u16 available[num];
+ *	__u16 avail_flags;
+ *	__u16 avail_idx;
  *	// Padding to the next align boundary.
  *	char pad[];
  *
@@ -94,13 +108,17 @@  struct vring {
  * };
  */
 static inline void vring_init(struct vring *vr, unsigned int num, void *p,
-			      unsigned long align)
+			      unsigned long align, bool avail_ring_first)
 {
+	struct vring_avail *avail = p + num * sizeof(struct vring_desc);
 	vr->num = num;
 	vr->desc = p;
-	vr->avail = p + num*sizeof(struct vring_desc);
-	vr->used = (void *)(((unsigned long)&vr->avail->ring[num] + align-1)
-			    & ~(align - 1));
+	vr->avail_ring = avail_ring_first ? (void*)avail : &avail->ring;
+	vr->avail = avail_ring_first ? (void *)&vr->avail_ring[num] : p;
+	vr->used = (void *)ALIGN((unsigned long)&avail->ring[num], align);
+	/* Verify that avail fits before used. */
+	BUG_ON((unsigned long)(vr->avail + 1) > (unsigned long)vr->used);
+	BUG_ON((unsigned long)(&vr->avail_ring[num]) > (unsigned long)vr->used);
 }
 
 static inline unsigned vring_size(unsigned int num, unsigned long align)