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Date: Wed, 7 Sep 2022 13:38:03 +0800
From: Jason Wang <jasowang@...hat.com>
To: Guo Zhi <qtxuning1999@...u.edu.cn>, eperezma@...hat.com,
sgarzare@...hat.com, mst@...hat.com
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [RFC v3 6/7] virtio: in order support for virtio_ring
在 2022/9/1 13:54, Guo Zhi 写道:
> If in order feature negotiated, we can skip the used ring to get
> buffer's desc id sequentially. For skipped buffers in the batch, the
> used ring doesn't contain the buffer length, actually there is not need
> to get skipped buffers' length as they are tx buffer.
>
> Signed-off-by: Guo Zhi <qtxuning1999@...u.edu.cn>
> ---
> drivers/virtio/virtio_ring.c | 74 +++++++++++++++++++++++++++++++-----
> 1 file changed, 64 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 00aa4b7a49c2..d52624179b43 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -103,6 +103,9 @@ struct vring_virtqueue {
> /* Host supports indirect buffers */
> bool indirect;
>
> + /* Host supports in order feature */
> + bool in_order;
> +
> /* Host publishes avail event idx */
> bool event;
>
> @@ -144,6 +147,19 @@ struct vring_virtqueue {
> /* DMA address and size information */
> dma_addr_t queue_dma_addr;
> size_t queue_size_in_bytes;
> +
> + /* If in_order feature is negotiated, here is the next head to consume */
> + u16 next_desc_begin;
> + /*
> + * If in_order feature is negotiated,
> + * here is the last descriptor's id in the batch
> + */
> + u16 last_desc_in_batch;
> + /*
> + * If in_order feature is negotiated,
> + * buffers except last buffer in the batch are skipped buffer
> + */
> + bool is_skipped_buffer;
> } split;
>
> /* Available for packed ring */
> @@ -584,8 +600,6 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> total_sg * sizeof(struct vring_desc),
> VRING_DESC_F_INDIRECT,
> false);
> - vq->split.desc_extra[head & (vq->split.vring.num - 1)].flags &=
> - ~VRING_DESC_F_NEXT;
This seems irrelevant.
> }
>
> /* We're using some buffers from the free list. */
> @@ -701,8 +715,16 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
> }
>
> vring_unmap_one_split(vq, i);
> - vq->split.desc_extra[i].next = vq->free_head;
> - vq->free_head = head;
> + /*
> + * If in_order feature is negotiated,
> + * the descriptors are made available in order.
> + * Since the free_head is already a circular list,
> + * it must consume it sequentially.
> + */
> + if (!vq->in_order) {
> + vq->split.desc_extra[i].next = vq->free_head;
> + vq->free_head = head;
> + }
>
> /* Plus final descriptor */
> vq->vq.num_free++;
> @@ -744,7 +766,7 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
> void *ret;
> - unsigned int i;
> + unsigned int i, j;
> u16 last_used;
>
> START_USE(vq);
> @@ -763,11 +785,38 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
> /* Only get used array entries after they have been exposed by host. */
> virtio_rmb(vq->weak_barriers);
>
> - last_used = (vq->last_used_idx & (vq->split.vring.num - 1));
> - i = virtio32_to_cpu(_vq->vdev,
> - vq->split.vring.used->ring[last_used].id);
> - *len = virtio32_to_cpu(_vq->vdev,
> - vq->split.vring.used->ring[last_used].len);
> + if (vq->in_order) {
> + last_used = (vq->last_used_idx & (vq->split.vring.num - 1));
Let's move this beyond the in_order check.
> + if (!vq->split.is_skipped_buffer) {
> + vq->split.last_desc_in_batch =
> + virtio32_to_cpu(_vq->vdev,
> + vq->split.vring.used->ring[last_used].id);
> + vq->split.is_skipped_buffer = true;
> + }
> + /* For skipped buffers in batch, we can ignore the len info, simply set len as 0 */
This seems to break the caller that depends on a correct len.
> + if (vq->split.next_desc_begin != vq->split.last_desc_in_batch) {
> + *len = 0;
> + } else {
> + *len = virtio32_to_cpu(_vq->vdev,
> + vq->split.vring.used->ring[last_used].len);
> + vq->split.is_skipped_buffer = false;
> + }
> + i = vq->split.next_desc_begin;
> + j = i;
> + /* Indirect only takes one descriptor in descriptor table */
> + while (!vq->indirect && (vq->split.desc_extra[j].flags & VRING_DESC_F_NEXT))
> + j = (j + 1) & (vq->split.vring.num - 1);
Any reason indirect descriptors can't be chained?
> + /* move to next */
> + j = (j + 1) % vq->split.vring.num;
> + /* Next buffer will use this descriptor in order */
> + vq->split.next_desc_begin = j;
Is it more efficient to poke the available ring?
Thanks
> + } else {
> + last_used = (vq->last_used_idx & (vq->split.vring.num - 1));
> + i = virtio32_to_cpu(_vq->vdev,
> + vq->split.vring.used->ring[last_used].id);
> + *len = virtio32_to_cpu(_vq->vdev,
> + vq->split.vring.used->ring[last_used].len);
> + }
>
> if (unlikely(i >= vq->split.vring.num)) {
> BAD_RING(vq, "id %u out of range\n", i);
> @@ -2223,6 +2272,7 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
>
> vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC) &&
> !context;
> + vq->in_order = virtio_has_feature(vdev, VIRTIO_F_IN_ORDER);
> vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
>
> if (virtio_has_feature(vdev, VIRTIO_F_ORDER_PLATFORM))
> @@ -2235,6 +2285,10 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
> vq->split.avail_flags_shadow = 0;
> vq->split.avail_idx_shadow = 0;
>
> + vq->split.next_desc_begin = 0;
> + vq->split.last_desc_in_batch = 0;
> + vq->split.is_skipped_buffer = false;
> +
> /* No callback? Tell other side not to bother us. */
> if (!callback) {
> vq->split.avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
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