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@xxxxxxxxxx>
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.
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