Re: [PATCH vhost v10 03/10] virtio_ring: split: support add premapped buf

From: Jason Wang <jasowang@redhat.com>
To: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc: virtualization@lists.linux-foundation.org,
	 "Michael S. Tsirkin" <mst@redhat.com>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	 Jakub Kicinski <kuba@kernel.org>,
	Paolo Abeni <pabeni@redhat.com>,
	Alexei Starovoitov <ast@kernel.org>,
	 Daniel Borkmann <daniel@iogearbox.net>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	 John Fastabend <john.fastabend@gmail.com>,
	netdev@vger.kernel.org, bpf@vger.kernel.org
Subject: Re: [PATCH vhost v10 03/10] virtio_ring: split: support add premapped buf
Date: Tue, 27 Jun 2023 16:03:26 +0800	[thread overview]
Message-ID: <CACGkMEtFiutSpM--2agR1YhS0MxreH4vFFAEdCaC6E8qxyjZ4g@mail.gmail.com> (raw)
In-Reply-To: <20230602092206.50108-4-xuanzhuo@linux.alibaba.com>

On Fri, Jun 2, 2023 at 5:22 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>
> If the vq is the premapped mode, use the sg_dma_address() directly.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
>  drivers/virtio/virtio_ring.c | 46 ++++++++++++++++++++++--------------
>  1 file changed, 28 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 2afdfb9e3e30..18212c3e056b 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -598,8 +598,12 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>                 for (sg = sgs[n]; sg; sg = sg_next(sg)) {
>                         dma_addr_t addr;
>
> -                       if (vring_map_one_sg(vq, sg, DMA_TO_DEVICE, &addr))
> -                               goto unmap_release;
> +                       if (vq->premapped) {
> +                               addr = sg_dma_address(sg);
> +                       } else {
> +                               if (vring_map_one_sg(vq, sg, DMA_TO_DEVICE, &addr))
> +                                       goto unmap_release;
> +                       }

Btw, I wonder whether or not it would be simple to implement the
vq->premapped check inside vring_map_one_sg() assuming the
!use_dma_api is done there as well.

>
>                         prev = i;
>                         /* Note that we trust indirect descriptor
> @@ -614,8 +618,12 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>                 for (sg = sgs[n]; sg; sg = sg_next(sg)) {
>                         dma_addr_t addr;
>
> -                       if (vring_map_one_sg(vq, sg, DMA_FROM_DEVICE, &addr))
> -                               goto unmap_release;
> +                       if (vq->premapped) {
> +                               addr = sg_dma_address(sg);
> +                       } else {
> +                               if (vring_map_one_sg(vq, sg, DMA_FROM_DEVICE, &addr))
> +                                       goto unmap_release;
> +                       }
>
>                         prev = i;
>                         /* Note that we trust indirect descriptor
> @@ -689,21 +697,23 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>         return 0;
>
>  unmap_release:
> -       err_idx = i;
> +       if (!vq->premapped) {

Can vq->premapped be true here? The label is named as "unmap_relase"
which implies "map" beforehand which seems not the case for
premapping.

Thanks

> +               err_idx = i;
>
> -       if (indirect)
> -               i = 0;
> -       else
> -               i = head;
> -
> -       for (n = 0; n < total_sg; n++) {
> -               if (i == err_idx)
> -                       break;
> -               if (indirect) {
> -                       vring_unmap_one_split_indirect(vq, &desc[i]);
> -                       i = virtio16_to_cpu(_vq->vdev, desc[i].next);
> -               } else
> -                       i = vring_unmap_one_split(vq, i);
> +               if (indirect)
> +                       i = 0;
> +               else
> +                       i = head;
> +
> +               for (n = 0; n < total_sg; n++) {
> +                       if (i == err_idx)
> +                               break;
> +                       if (indirect) {
> +                               vring_unmap_one_split_indirect(vq, &desc[i]);
> +                               i = virtio16_to_cpu(_vq->vdev, desc[i].next);
> +                       } else
> +                               i = vring_unmap_one_split(vq, i);
> +               }
>         }
>
>         if (indirect)
> --
> 2.32.0.3.g01195cf9f
>