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Message-ID: <20230622153111-mutt-send-email-mst@kernel.org>
Date: Thu, 22 Jun 2023 15:36:41 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: virtualization@...ts.linux-foundation.org,
Jason Wang <jasowang@...hat.com>,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Jesper Dangaard Brouer <hawk@...nel.org>,
John Fastabend <john.fastabend@...il.com>, netdev@...r.kernel.org,
bpf@...r.kernel.org
Subject: Re: [PATCH vhost v10 05/10] virtio_ring: split-detach: support
return dma info to driver
On Fri, Jun 02, 2023 at 05:22:01PM +0800, Xuan Zhuo wrote:
> Under the premapped mode, the driver needs to unmap the DMA address
> after receiving the buffer. The virtio core records the DMA address,
> so the driver needs a way to get the dma info from the virtio core.
>
> A straightforward approach is to pass an array to the virtio core when
> calling virtqueue_get_buf(). However, it is not feasible when there are
> multiple DMA addresses in the descriptor chain, and the array size is
> unknown.
>
> To solve this problem, a helper be introduced. After calling
> virtqueue_get_buf(), the driver can call the helper to
> retrieve a dma info. If the helper function returns -EAGAIN, it means
> that there are more DMA addresses to be processed, and the driver should
> call the helper function again. To keep track of the current position in
> the chain, a cursor must be passed to the helper function, which is
> initialized by virtqueue_get_buf().
>
> Some processes are done inside this helper, so this helper MUST be
> called under the premapped mode.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> ---
> drivers/virtio/virtio_ring.c | 118 ++++++++++++++++++++++++++++++++---
> include/linux/virtio.h | 11 ++++
> 2 files changed, 119 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index dc109fbc05a5..cdc4349f6066 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -754,8 +754,95 @@ static bool virtqueue_kick_prepare_split(struct virtqueue *_vq)
> return needs_kick;
> }
>
> -static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
> - void **ctx)
> +static void detach_cursor_init_split(struct vring_virtqueue *vq,
> + struct virtqueue_detach_cursor *cursor, u16 head)
> +{
> + struct vring_desc_extra *extra;
> +
> + extra = &vq->split.desc_extra[head];
> +
> + /* Clear data ptr. */
> + vq->split.desc_state[head].data = NULL;
> +
> + cursor->head = head;
> + cursor->done = 0;
> +
> + if (extra->flags & VRING_DESC_F_INDIRECT) {
> + cursor->num = extra->len / sizeof(struct vring_desc);
> + cursor->indirect = true;
> + cursor->pos = 0;
> +
> + vring_unmap_one_split(vq, head);
> +
> + extra->next = vq->free_head;
> +
> + vq->free_head = head;
> +
> + /* Plus final descriptor */
> + vq->vq.num_free++;
> +
> + } else {
> + cursor->indirect = false;
> + cursor->pos = head;
> + }
> +}
> +
> +static int virtqueue_detach_split(struct virtqueue *_vq, struct virtqueue_detach_cursor *cursor,
> + dma_addr_t *addr, u32 *len, enum dma_data_direction *dir)
> +{
I don't get it. This is generic split vq code? Why is it unconditionally
wasting time with cursors etc? Poking at split.desc_extra when not
necessary is also not really nice, will cause lots of cache misses.
And it looks like we duplicated a bunch of logic?
> + struct vring_virtqueue *vq = to_vvq(_vq);
> + __virtio16 nextflag = cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_NEXT);
> + int rc = -EAGAIN;
> +
> + if (unlikely(cursor->done))
> + return -EINVAL;
> +
> + if (!cursor->indirect) {
> + struct vring_desc_extra *extra;
> + unsigned int i;
> +
> + i = cursor->pos;
> +
> + extra = &vq->split.desc_extra[i];
> +
> + if (vq->split.vring.desc[i].flags & nextflag) {
> + cursor->pos = extra->next;
> + } else {
> + extra->next = vq->free_head;
> + vq->free_head = cursor->head;
> + cursor->done = true;
> + rc = 0;
> + }
> +
> + *addr = extra->addr;
> + *len = extra->len;
> + *dir = (extra->flags & VRING_DESC_F_WRITE) ? DMA_FROM_DEVICE : DMA_TO_DEVICE;
> +
> + vq->vq.num_free++;
> +
> + } else {
> + struct vring_desc *indir_desc, *desc;
> + u16 flags;
> +
> + indir_desc = vq->split.desc_state[cursor->head].indir_desc;
> + desc = &indir_desc[cursor->pos];
> +
> + flags = virtio16_to_cpu(vq->vq.vdev, desc->flags);
> + *addr = virtio64_to_cpu(vq->vq.vdev, desc->addr);
> + *len = virtio32_to_cpu(vq->vq.vdev, desc->len);
> + *dir = (flags & VRING_DESC_F_WRITE) ? DMA_FROM_DEVICE : DMA_TO_DEVICE;
> +
> + if (++cursor->pos == cursor->num) {
> + kfree(indir_desc);
> + cursor->done = true;
> + return 0;
> + }
> + }
> +
> + return rc;
> +}
> +
> +static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head)
> {
> unsigned int i, j;
> __virtio16 nextflag = cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_NEXT);
> @@ -799,8 +886,6 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
>
> kfree(indir_desc);
> vq->split.desc_state[head].indir_desc = NULL;
> - } else if (ctx) {
> - *ctx = vq->split.desc_state[head].indir_desc;
> }
> }
>
> @@ -812,7 +897,8 @@ static bool more_used_split(const struct vring_virtqueue *vq)
>
> static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
> unsigned int *len,
> - void **ctx)
> + void **ctx,
> + struct virtqueue_detach_cursor *cursor)
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
> void *ret;
> @@ -852,7 +938,15 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
>
> /* detach_buf_split clears data, so grab it now. */
> ret = vq->split.desc_state[i].data;
> - detach_buf_split(vq, i, ctx);
> +
> + if (!vq->indirect && ctx)
> + *ctx = vq->split.desc_state[i].indir_desc;
> +
> + if (vq->premapped)
> + detach_cursor_init_split(vq, cursor, i);
> + else
> + detach_buf_split(vq, i);
> +
> vq->last_used_idx++;
> /* If we expect an interrupt for the next entry, tell host
> * by writing event index and flush out the write before
> @@ -961,7 +1055,8 @@ static bool virtqueue_enable_cb_delayed_split(struct virtqueue *_vq)
> return true;
> }
>
> -static void *virtqueue_detach_unused_buf_split(struct virtqueue *_vq)
> +static void *virtqueue_detach_unused_buf_split(struct virtqueue *_vq,
> + struct virtqueue_detach_cursor *cursor)
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
> unsigned int i;
> @@ -974,7 +1069,10 @@ static void *virtqueue_detach_unused_buf_split(struct virtqueue *_vq)
> continue;
> /* detach_buf_split clears data, so grab it now. */
> buf = vq->split.desc_state[i].data;
> - detach_buf_split(vq, i, NULL);
> + if (vq->premapped)
> + detach_cursor_init_split(vq, cursor, i);
> + else
> + detach_buf_split(vq, i);
> vq->split.avail_idx_shadow--;
> vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev,
> vq->split.avail_idx_shadow);
> @@ -2361,7 +2459,7 @@ void *virtqueue_get_buf_ctx(struct virtqueue *_vq, unsigned int *len,
> struct vring_virtqueue *vq = to_vvq(_vq);
>
> return vq->packed_ring ? virtqueue_get_buf_ctx_packed(_vq, len, ctx) :
> - virtqueue_get_buf_ctx_split(_vq, len, ctx);
> + virtqueue_get_buf_ctx_split(_vq, len, ctx, NULL);
> }
> EXPORT_SYMBOL_GPL(virtqueue_get_buf_ctx);
>
> @@ -2493,7 +2591,7 @@ void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
> struct vring_virtqueue *vq = to_vvq(_vq);
>
> return vq->packed_ring ? virtqueue_detach_unused_buf_packed(_vq) :
> - virtqueue_detach_unused_buf_split(_vq);
> + virtqueue_detach_unused_buf_split(_vq, NULL);
> }
> EXPORT_SYMBOL_GPL(virtqueue_detach_unused_buf);
>
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index 1fc0e1023bd4..eb4a4e4329aa 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -38,6 +38,17 @@ struct virtqueue {
> void *priv;
> };
>
> +struct virtqueue_detach_cursor {
> + unsigned indirect:1;
> + unsigned done:1;
> + unsigned hole:14;
> +
> + /* for split head */
> + unsigned head:16;
> + unsigned num:16;
> + unsigned pos:16;
> +};
> +
is cursor ever stored somewhere? If not don't use bitfields,
they cause many gcc versions to generate atrocious code.
> int virtqueue_add_outbuf(struct virtqueue *vq,
> struct scatterlist sg[], unsigned int num,
> void *data,
> --
> 2.32.0.3.g01195cf9f
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