[Qemu-devel] Re: [PATCHv3 1/2] virtio: support layout with avail ring before idx

[Rusty Russell <rusty@rustcorp.com.au>]

On Wed, 2 Jun 2010 12:17:12 am Michael S. Tsirkin wrote:
> This adds an (unused) option to put available ring before control (avail
> index, flags), and adds padding between index and flags. This avoids
> cache line sharing between control and ring, and also makes it possible
> to extend avail control without incurring extra cache misses.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

No no no no.  254?  You're trying to Morton me![1]

How's this (untested):

diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -74,8 +74,8 @@ struct vring {
 /* The standard layout for the ring is a continuous chunk of memory which looks
  * like this.  We assume num is a power of 2.
  *
- * struct vring
- * {
+ * struct vring {
+ *	*** The driver writes to this part.
  *	// The actual descriptors (16 bytes each)
  *	struct vring_desc desc[num];
  *
@@ -84,9 +84,11 @@ struct vring {
  *	__u16 avail_idx;
  *	__u16 available[num];
  *
- *	// Padding to the next align boundary.
+ *	// Padding so used_flags is on the next align boundary.
  *	char pad[];
+ *	__u16 last_used; // On a cacheline of its own.
  *
+ *	*** The device writes to this part.
  *	// A ring of used descriptor heads with free-running index.
  *	__u16 used_flags;
  *	__u16 used_idx;
@@ -110,6 +112,12 @@ static inline unsigned vring_size(unsign
 		+ sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num;
 }
 
+/* Last used index sits at the very end of the driver part of the struct */
+static inline __u16 *vring_last_used_idx(const struct vring *vr)
+{
+	return (__u16 *)vr->used - 1;
+}
+
 #ifdef __KERNEL__
 #include <linux/irqreturn.h>
 struct virtio_device;

Cheers,
Rusty.
[1] Andrew Morton has this technique where he posts a solution so ugly it
    forces others to fix it properly.  Ego-roping, basically.