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SubjectRe: [PATCH net-next v2 4/5] virtio_ring: add event idx support in packed ring
On Wed, Jul 11, 2018 at 10:27:10AM +0800, Tiwei Bie wrote:
> This commit introduces the EVENT_IDX support in packed ring.
>
> Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>

Besides the usual comment about hard-coded constants like <<15:
does this actually do any good for performance? We don't
have to if we do not want to.

> ---
> drivers/virtio/virtio_ring.c | 73 ++++++++++++++++++++++++++++++++----
> 1 file changed, 65 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index f317b485ba54..f79a1e17f7d1 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -1050,7 +1050,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
> - u16 flags;
> + u16 new, old, off_wrap, flags, wrap_counter, event_idx;
> bool needs_kick;
> u32 snapshot;
>
> @@ -1059,9 +1059,19 @@ static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
> * suppressions. */
> virtio_mb(vq->weak_barriers);
>
> + old = vq->next_avail_idx - vq->num_added;
> + new = vq->next_avail_idx;
> + vq->num_added = 0;
> +
> snapshot = READ_ONCE(*(u32 *)vq->vring_packed.device);
> + off_wrap = virtio16_to_cpu(_vq->vdev, (__virtio16)(snapshot & 0xffff));
> flags = virtio16_to_cpu(_vq->vdev, (__virtio16)(snapshot >> 16)) & 0x3;


some kind of struct union would be helpful to make this readable.

>
> + wrap_counter = off_wrap >> 15;
> + event_idx = off_wrap & ~(1 << 15);
> + if (wrap_counter != vq->avail_wrap_counter)
> + event_idx -= vq->vring_packed.num;
> +
> #ifdef DEBUG
> if (vq->last_add_time_valid) {
> WARN_ON(ktime_to_ms(ktime_sub(ktime_get(),
> @@ -1070,7 +1080,10 @@ static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
> vq->last_add_time_valid = false;
> #endif
>
> - needs_kick = (flags != VRING_EVENT_F_DISABLE);
> + if (flags == VRING_EVENT_F_DESC)
> + needs_kick = vring_need_event(event_idx, new, old);
> + else
> + needs_kick = (flags != VRING_EVENT_F_DISABLE);
> END_USE(vq);
> return needs_kick;
> }
> @@ -1185,6 +1198,15 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
> ret = vq->desc_state_packed[id].data;
> detach_buf_packed(vq, id, ctx);
>
> + /* If we expect an interrupt for the next entry, tell host
> + * by writing event index and flush out the write before
> + * the read in the next get_buf call. */
> + if (vq->event_flags_shadow == VRING_EVENT_F_DESC)
> + virtio_store_mb(vq->weak_barriers,
> + &vq->vring_packed.driver->off_wrap,
> + cpu_to_virtio16(_vq->vdev, vq->last_used_idx |
> + ((u16)vq->used_wrap_counter << 15)));
> +
> #ifdef DEBUG
> vq->last_add_time_valid = false;
> #endif
> @@ -1213,8 +1235,18 @@ static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
> /* We optimistically turn back on interrupts, then check if there was
> * more to do. */
>
> + if (vq->event) {
> + vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
> + vq->last_used_idx |
> + ((u16)vq->used_wrap_counter << 15));
> + /* We need to update event offset and event wrap
> + * counter first before updating event flags. */
> + virtio_wmb(vq->weak_barriers);
> + }
> +
> if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> - vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
> + vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC :
> + VRING_EVENT_F_ENABLE;
> vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> vq->event_flags_shadow);
> }
> @@ -1238,22 +1270,47 @@ static bool virtqueue_poll_packed(struct virtqueue *_vq, unsigned off_wrap)
> static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
> + u16 bufs, used_idx, wrap_counter;
>
> START_USE(vq);
>
> /* We optimistically turn back on interrupts, then check if there was
> * more to do. */
>
> + if (vq->event) {
> + /* TODO: tune this threshold */
> + bufs = (vq->vring_packed.num - _vq->num_free) * 3 / 4;
> + wrap_counter = vq->used_wrap_counter;
> +
> + used_idx = vq->last_used_idx + bufs;
> + if (used_idx >= vq->vring_packed.num) {
> + used_idx -= vq->vring_packed.num;
> + wrap_counter ^= 1;
> + }
> +
> + vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
> + used_idx | (wrap_counter << 15));
> +
> + /* We need to update event offset and event wrap
> + * counter first before updating event flags. */
> + virtio_wmb(vq->weak_barriers);
> + } else {
> + used_idx = vq->last_used_idx;
> + wrap_counter = vq->used_wrap_counter;
> + }
> +
> if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> - vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
> + vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC :
> + VRING_EVENT_F_ENABLE;
> vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> vq->event_flags_shadow);
> - /* We need to enable interrupts first before re-checking
> - * for more used buffers. */
> - virtio_mb(vq->weak_barriers);
> }
>
> - if (more_used_packed(vq)) {
> + /* We need to update event suppression structure first
> + * before re-checking for more used buffers. */
> + virtio_mb(vq->weak_barriers);
> +

mb is expensive. We should not do it if we changed nothing.

> + if (is_used_desc_packed(vq, used_idx, wrap_counter)) {
> END_USE(vq);
> return false;
> }
> --
> 2.18.0

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