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SubjectRe: virtio: put last_used and last_avail index into ring itself.
> virtio: put last_used and last_avail index into ring itself.
>
> Generally, the other end of the virtio ring doesn't need to see where
> you're up to in consuming the ring. However, to completely understand
> what's going on from the outside, this information must be exposed.
> For example, if you want to save and restore a virtio_ring, but you're
> not the consumer because the kernel is using it directly.
>
> Fortunately, we have room to expand: the ring is always a whole number
> of pages and there's hundreds of bytes of padding after the avail ring
> and the used ring, whatever the number of descriptors (which must be a
> power of 2).
>
> We add a feature bit so the guest can tell the host that it's writing
> out the current value there, if it wants to use that.
>
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

I've been looking at this patch some more (more on why
later), and I wonder: would it be better to add some
alignment to the last used index address, so that
if we later add more stuff at the tail, it all
fits in a single cache line?

We use a new feature bit anyway, so layout change should not be
a problem.

Since I raised the question of caches: for used ring,
the ring is not aligned to 64 bit, so on CPUs with 64 bit
or larger cache lines, used entries will often cross
cache line boundaries. Am I right and might it
have been better to align ring entries to cache line boundaries?

What do you think?

> ---
> drivers/virtio/virtio_ring.c | 23 +++++++++++++++--------
> include/linux/virtio_ring.h | 12 +++++++++++-
> 2 files changed, 26 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -71,9 +71,6 @@ struct vring_virtqueue
> /* Number we've added since last sync. */
> unsigned int num_added;
>
> - /* Last used index we've seen. */
> - u16 last_used_idx;
> -
> /* How to notify other side. FIXME: commonalize hcalls! */
> void (*notify)(struct virtqueue *vq);
>
> @@ -278,12 +275,13 @@ static void detach_buf(struct vring_virt
>
> static inline bool more_used(const struct vring_virtqueue *vq)
> {
> - return vq->last_used_idx != vq->vring.used->idx;
> + return vring_last_used(&vq->vring) != vq->vring.used->idx;
> }
>
> static void *vring_get_buf(struct virtqueue *_vq, unsigned int *len)
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
> + struct vring_used_elem *u;
> void *ret;
> unsigned int i;
>
> @@ -300,8 +298,11 @@ static void *vring_get_buf(struct virtqu
> return NULL;
> }
>
> - i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id;
> - *len = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].len;
> + u = &vq->vring.used->ring[vring_last_used(&vq->vring) % vq->vring.num];
> + i = u->id;
> + *len = u->len;
> + /* Make sure we don't reload i after doing checks. */
> + rmb();
>
> if (unlikely(i >= vq->vring.num)) {
> BAD_RING(vq, "id %u out of range\n", i);
> @@ -315,7 +316,8 @@ static void *vring_get_buf(struct virtqu
> /* detach_buf clears data, so grab it now. */
> ret = vq->data[i];
> detach_buf(vq, i);
> - vq->last_used_idx++;
> + vring_last_used(&vq->vring)++;
> +
> END_USE(vq);
> return ret;
> }
> @@ -402,7 +404,6 @@ struct virtqueue *vring_new_virtqueue(un
> vq->vq.name = name;
> vq->notify = notify;
> vq->broken = false;
> - vq->last_used_idx = 0;
> vq->num_added = 0;
> list_add_tail(&vq->vq.list, &vdev->vqs);
> #ifdef DEBUG
> @@ -413,6 +414,10 @@ struct virtqueue *vring_new_virtqueue(un
>
> vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC);
>
> + /* We publish indices whether they offer it or not: if not, it's junk
> + * space anyway. But calling this acknowledges the feature. */
> + virtio_has_feature(vdev, VIRTIO_RING_F_PUBLISH_INDICES);
> +
> /* No callback? Tell other side not to bother us. */
> if (!callback)
> vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;
> @@ -443,6 +448,8 @@ void vring_transport_features(struct vir
> switch (i) {
> case VIRTIO_RING_F_INDIRECT_DESC:
> break;
> + case VIRTIO_RING_F_PUBLISH_INDICES:
> + break;
> default:
> /* We don't understand this bit. */
> clear_bit(i, vdev->features);
> 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
> @@ -29,6 +29,9 @@
> /* We support indirect buffer descriptors */
> #define VIRTIO_RING_F_INDIRECT_DESC 28
>
> +/* We publish our last-seen used index at the end of the avail ring. */
> +#define VIRTIO_RING_F_PUBLISH_INDICES 29
> +
> /* Virtio ring descriptors: 16 bytes. These can chain together via "next". */
> struct vring_desc
> {
> @@ -87,6 +90,7 @@ struct vring {
> * __u16 avail_flags;
> * __u16 avail_idx;
> * __u16 available[num];
> + * __u16 last_used_idx;
> *
> * // Padding to the next align boundary.
> * char pad[];
> @@ -95,6 +99,7 @@ struct vring {
> * __u16 used_flags;
> * __u16 used_idx;
> * struct vring_used_elem used[num];
> + * __u16 last_avail_idx;
> * };
> */
> static inline void vring_init(struct vring *vr, unsigned int num, void *p,
> @@ -111,9 +116,14 @@ static inline unsigned vring_size(unsign
> {
> return ((sizeof(struct vring_desc) * num + sizeof(__u16) * (2 + num)
> + align - 1) & ~(align - 1))
> - + sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num;
> + + sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num + 2;
> }
>
> +/* We publish the last-seen used index at the end of the available ring, and
> + * vice-versa. These are at the end for backwards compatibility. */
> +#define vring_last_used(vr) ((vr)->avail->ring[(vr)->num])
> +#define vring_last_avail(vr) (*(__u16 *)&(vr)->used->ring[(vr)->num])
> +
> #ifdef __KERNEL__
> #include <linux/irqreturn.h>
> struct virtio_device;


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