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SubjectRe: [RFC v4 4/5] virtio_ring: add event idx support in packed ring
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On 2018年05月16日 16:37, Tiwei Bie wrote:
> This commit introduces the event idx support in
> packed ring.
>
> Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
> ---
> drivers/virtio/virtio_ring.c | 75 +++++++++++++++++++++++++++++++++---
> 1 file changed, 70 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index c6c5deb0e3ae..de3839f3621a 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -1006,7 +1006,7 @@ 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;
>
> @@ -1015,9 +1015,19 @@ 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 = *(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;
>
> + wrap_counter = off_wrap >> 15;
> + event_idx = off_wrap & ~(1<<15);
> + if (wrap_counter != vq->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(),
> @@ -1026,7 +1036,10 @@ 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;
> }
> @@ -1098,7 +1111,7 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
> void **ctx)
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
> - u16 last_used, id;
> + u16 wrap_counter, last_used, id;
> void *ret;
>
> START_USE(vq);
> @@ -1138,6 +1151,19 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
> ret = vq->desc_state[id].data;
> detach_buf_packed(vq, last_used, id, ctx);
>
> + wrap_counter = vq->wrap_counter;
> + if (vq->last_used_idx > vq->next_avail_idx)
> + wrap_counter ^= 1;
> +
> + /* 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 |
> + (wrap_counter << 15)));
> +
> #ifdef DEBUG
> vq->last_add_time_valid = false;
> #endif
> @@ -1160,15 +1186,27 @@ static void virtqueue_disable_cb_packed(struct virtqueue *_vq)
> static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
> + u16 wrap_counter;
>
> START_USE(vq);
>
> /* We optimistically turn back on interrupts, then check if there was
> * more to do. */
> + /* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
> + * either clear the flags bit or point the event index at the next
> + * entry. Always update the event index to keep code simple. */
> +
> + wrap_counter = vq->wrap_counter;
> + if (vq->last_used_idx > vq->next_avail_idx)

Should this be ">=" consider rx refill may try to completely fill the ring?

> + wrap_counter ^= 1;
> +
> + vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
> + vq->last_used_idx | (wrap_counter << 15));
>
> if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> virtio_wmb(vq->weak_barriers);
> - 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);
> }
> @@ -1194,15 +1232,40 @@ static bool virtqueue_poll_packed(struct virtqueue *_vq, unsigned last_used_idx)
> 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. */
> + /* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
> + * either clear the flags bit or point the event index at the next
> + * entry. Always update the event index to keep code simple. */
> +
> + /* TODO: tune this threshold */
> + if (vq->next_avail_idx < vq->last_used_idx)
> + bufs = (vq->vring_packed.num + vq->next_avail_idx -
> + vq->last_used_idx) * 3 / 4;
> + else
> + bufs = (vq->next_avail_idx - vq->last_used_idx) * 3 / 4;
> +
> + wrap_counter = vq->wrap_counter;
> + if (vq->last_used_idx > vq->next_avail_idx)
> + wrap_counter ^= 1;
> +
> + used_idx = vq->last_used_idx + bufs;
> + if (used_idx >= vq->vring_packed.num) {
> + used_idx -= vq->vring_packed.num;
> + wrap_counter ^= 1;
> + }

Looks correct but maybe it's better to add some comments for such logic.

> +
> + vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
> + used_idx | (wrap_counter << 15));
>
> if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> virtio_wmb(vq->weak_barriers);
> - 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);
> }
> @@ -1869,8 +1932,10 @@ void vring_transport_features(struct virtio_device *vdev)
> switch (i) {
> case VIRTIO_RING_F_INDIRECT_DESC:
> break;
> +#if 0
> case VIRTIO_RING_F_EVENT_IDX:
> break;
> +#endif

Maybe it's time to remove this #if 0.

Thanks

> case VIRTIO_F_VERSION_1:
> break;
> case VIRTIO_F_IOMMU_PLATFORM:

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