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Date: Thu, 21 Mar 2024 13:57:06 +0800
From: Jason Wang <jasowang@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: virtualization@...ts.linux.dev, "Michael S. Tsirkin" <mst@...hat.com>,
"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Subject: Re: [PATCH vhost v4 02/10] virtio_ring: packed: remove double check
of the unmap ops
On Tue, Mar 12, 2024 at 11:36 AM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> In the functions vring_unmap_extra_packed and vring_unmap_desc_packed,
> multiple checks are made whether unmap is performed and whether it is
> INDIRECT.
>
> These two functions are usually called in a loop, and we should put the
> check outside the loop.
>
> And we unmap the descs with VRING_DESC_F_INDIRECT on the same path with
> other descs, that make the thing more complex. If we distinguish the
> descs with VRING_DESC_F_INDIRECT before unmap, thing will be clearer.
>
> 1. only one desc of the desc table is used, we do not need the loop
> 2. the called unmap api is difference from the other desc
> 3. the vq->premapped is not needed to check
> 4. the vq->indirect is not needed to check
> 5. the state->indir_desc must not be null
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> ---
> drivers/virtio/virtio_ring.c | 78 ++++++++++++++++++------------------
> 1 file changed, 40 insertions(+), 38 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index c2779e34aac7..0dfbd17e5a87 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -1214,6 +1214,7 @@ static u16 packed_last_used(u16 last_used_idx)
> return last_used_idx & ~(-(1 << VRING_PACKED_EVENT_F_WRAP_CTR));
> }
>
> +/* caller must check vring_need_unmap_buffer() */
> static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
> const struct vring_desc_extra *extra)
> {
> @@ -1221,33 +1222,18 @@ static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
>
> flags = extra->flags;
>
> - if (flags & VRING_DESC_F_INDIRECT) {
> - if (!vq->use_dma_api)
> - return;
> -
> - dma_unmap_single(vring_dma_dev(vq),
> - extra->addr, extra->len,
> - (flags & VRING_DESC_F_WRITE) ?
> - DMA_FROM_DEVICE : DMA_TO_DEVICE);
> - } else {
> - if (!vring_need_unmap_buffer(vq))
> - return;
> -
> - dma_unmap_page(vring_dma_dev(vq),
> - extra->addr, extra->len,
> - (flags & VRING_DESC_F_WRITE) ?
> - DMA_FROM_DEVICE : DMA_TO_DEVICE);
> - }
> + dma_unmap_page(vring_dma_dev(vq),
> + extra->addr, extra->len,
> + (flags & VRING_DESC_F_WRITE) ?
> + DMA_FROM_DEVICE : DMA_TO_DEVICE);
> }
>
> +/* caller must check vring_need_unmap_buffer() */
> static void vring_unmap_desc_packed(const struct vring_virtqueue *vq,
> const struct vring_packed_desc *desc)
> {
> u16 flags;
>
> - if (!vring_need_unmap_buffer(vq))
> - return;
> -
> flags = le16_to_cpu(desc->flags);
>
> dma_unmap_page(vring_dma_dev(vq),
> @@ -1323,7 +1309,7 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> total_sg * sizeof(struct vring_packed_desc),
> DMA_TO_DEVICE);
> if (vring_mapping_error(vq, addr)) {
> - if (vq->premapped)
> + if (!vring_need_unmap_buffer(vq))
> goto free_desc;
>
> goto unmap_release;
> @@ -1338,10 +1324,11 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> vq->packed.desc_extra[id].addr = addr;
> vq->packed.desc_extra[id].len = total_sg *
> sizeof(struct vring_packed_desc);
> - vq->packed.desc_extra[id].flags = VRING_DESC_F_INDIRECT |
> - vq->packed.avail_used_flags;
> }
>
> + vq->packed.desc_extra[id].flags = VRING_DESC_F_INDIRECT |
> + vq->packed.avail_used_flags;
> +
> /*
> * A driver MUST NOT make the first descriptor in the list
> * available before all subsequent descriptors comprising
> @@ -1382,6 +1369,8 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> unmap_release:
> err_idx = i;
>
> + WARN_ON(!vring_need_unmap_buffer(vq));
> +
> for (i = 0; i < err_idx; i++)
> vring_unmap_desc_packed(vq, &desc[i]);
>
> @@ -1475,12 +1464,13 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> desc[i].len = cpu_to_le32(sg->length);
> desc[i].id = cpu_to_le16(id);
>
> - if (unlikely(vq->use_dma_api)) {
> + if (vring_need_unmap_buffer(vq)) {
> vq->packed.desc_extra[curr].addr = addr;
> vq->packed.desc_extra[curr].len = sg->length;
> - vq->packed.desc_extra[curr].flags =
> - le16_to_cpu(flags);
> }
> +
> + vq->packed.desc_extra[curr].flags = le16_to_cpu(flags);
> +
> prev = curr;
> curr = vq->packed.desc_extra[curr].next;
>
> @@ -1530,6 +1520,8 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
>
> vq->packed.avail_used_flags = avail_used_flags;
>
> + WARN_ON(!vring_need_unmap_buffer(vq));
> +
> for (n = 0; n < total_sg; n++) {
> if (i == err_idx)
> break;
> @@ -1599,7 +1591,9 @@ static void detach_buf_packed(struct vring_virtqueue *vq,
> struct vring_desc_state_packed *state = NULL;
> struct vring_packed_desc *desc;
> unsigned int i, curr;
> + u16 flags;
>
> + flags = vq->packed.desc_extra[id].flags;
> state = &vq->packed.desc_state[id];
>
> /* Clear data ptr. */
> @@ -1609,22 +1603,32 @@ static void detach_buf_packed(struct vring_virtqueue *vq,
> vq->free_head = id;
> vq->vq.num_free += state->num;
>
> - if (unlikely(vq->use_dma_api)) {
> - curr = id;
> - for (i = 0; i < state->num; i++) {
> - vring_unmap_extra_packed(vq,
> - &vq->packed.desc_extra[curr]);
> - curr = vq->packed.desc_extra[curr].next;
> + if (!(flags & VRING_DESC_F_INDIRECT)) {
> + if (vring_need_unmap_buffer(vq)) {
> + curr = id;
> + for (i = 0; i < state->num; i++) {
> + vring_unmap_extra_packed(vq,
> + &vq->packed.desc_extra[curr]);
> + curr = vq->packed.desc_extra[curr].next;
> + }
> }
> - }
>
> - if (vq->indirect) {
> + if (ctx)
> + *ctx = state->indir_desc;
> + } else {
> + const struct vring_desc_extra *extra;
> u32 len;
>
> + if (vq->use_dma_api) {
> + extra = &vq->packed.desc_extra[id];
> + dma_unmap_single(vring_dma_dev(vq),
> + extra->addr, extra->len,
> + (flags & VRING_DESC_F_WRITE) ?
> + DMA_FROM_DEVICE : DMA_TO_DEVICE);
> + }
Theoretically, indirect descriptors could be chained. It is supported
without this patch but not here.
Thanks
> +
> /* Free the indirect table, if any, now that it's unmapped. */
> desc = state->indir_desc;
> - if (!desc)
> - return;
>
> if (vring_need_unmap_buffer(vq)) {
> len = vq->packed.desc_extra[id].len;
> @@ -1634,8 +1638,6 @@ static void detach_buf_packed(struct vring_virtqueue *vq,
> }
> kfree(desc);
> state->indir_desc = NULL;
> - } else if (ctx) {
> - *ctx = state->indir_desc;
> }
> }
>
> --
> 2.32.0.3.g01195cf9f
>
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