On Fri, Jun 04, 2010 at 12:04:57PM +0930, Rusty Russell wrote:
> 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]
Hmm, I wonder what will we do if we want a 3rd field on
a separate chacheline. But ok.
> How's this (untested):
I think we also want to put flags there as well,
they are used on interrupt path, together with last used index.
> 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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