Re: [PATCH net-next 04/13] virtio_ring: perform premapped operations based on per-buffer

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On Tue, Aug 20, 2024 at 3:33 PM Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> wrote:
>
> The current configuration sets the virtqueue (vq) to premapped mode,
> implying that all buffers submitted to this queue must be mapped ahead
> of time. This presents a challenge for the virtnet send queue (sq): the
> virtnet driver would be required to keep track of dma information for vq
> size * 17, which can be substantial. However, if the premapped mode were
> applied on a per-buffer basis, the complexity would be greatly reduced.
> With AF_XDP enabled, AF_XDP buffers would become premapped, while kernel
> skb buffers could remain unmapped.

Is this only applied to TX or both TX and RX.

>
> We can distinguish them by sg_page(sg), When sg_page(sg) is NULL, this
> indicates that the driver has performed DMA mapping in advance, allowing
> the Virtio core to directly utilize sg_dma_address(sg) without
> conducting any internal DMA mapping.

This seems conflict with the code below?

#define sg_is_premapped(sg) (!sg_page(sg))

And rethink of the design, a question is that is there a chance that
VM_PFNMAP area could be used for virtio-net? If it is true, the trick
for sg_page() can not work?

A quick glance told me AF_XEP seems to be safe as it uses
pin_user_pages(), but we need to check other possibilities.

Or we need to fall back to our previous idea, having new APIs.

> Additionally, DMA unmap operations
> for this buffer will be bypassed.
>
> Suggested-by: Jason Wang <jasowang@xxxxxxxxxx>
> Signed-off-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> ---
>  drivers/virtio/virtio_ring.c | 70 +++++++++++++++++++++---------------
>  1 file changed, 41 insertions(+), 29 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index b43eca93015c..7efddc71af67 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -235,6 +235,7 @@ static void vring_free(struct virtqueue *_vq);
>   */
>
>  #define to_vvq(_vq) container_of_const(_vq, struct vring_virtqueue, vq)
> +#define sg_is_premapped(sg) (!sg_page(sg))
>
>  static bool virtqueue_use_indirect(const struct vring_virtqueue *vq,
>                                    unsigned int total_sg)
> @@ -292,9 +293,10 @@ static bool vring_use_dma_api(const struct virtio_device *vdev)
>         return false;
>  }
>
> -static bool vring_need_unmap_buffer(const struct vring_virtqueue *vring)
> +static bool vring_need_unmap_buffer(const struct vring_virtqueue *vring,
> +                                   const struct vring_desc_extra *extra)
>  {
> -       return vring->use_dma_api && !vring->premapped;
> +       return vring->use_dma_api && (extra->addr != DMA_MAPPING_ERROR);
>  }
>
>  size_t virtio_max_dma_size(const struct virtio_device *vdev)
> @@ -366,7 +368,7 @@ static struct device *vring_dma_dev(const struct vring_virtqueue *vq)
>  static int vring_map_one_sg(const struct vring_virtqueue *vq, struct scatterlist *sg,
>                             enum dma_data_direction direction, dma_addr_t *addr)
>  {
> -       if (vq->premapped) {
> +       if (sg_is_premapped(sg)) {
>                 *addr = sg_dma_address(sg);
>                 return 0;
>         }
> @@ -457,7 +459,7 @@ static unsigned int vring_unmap_one_split(const struct vring_virtqueue *vq,
>                                  (flags & VRING_DESC_F_WRITE) ?
>                                  DMA_FROM_DEVICE : DMA_TO_DEVICE);
>         } else {
> -               if (!vring_need_unmap_buffer(vq))
> +               if (!vring_need_unmap_buffer(vq, extra))
>                         goto out;
>
>                 dma_unmap_page(vring_dma_dev(vq),
> @@ -510,7 +512,7 @@ static inline unsigned int virtqueue_add_desc_split(struct virtqueue *vq,
>                                                     dma_addr_t addr,
>                                                     unsigned int len,
>                                                     u16 flags,
> -                                                   bool indirect)
> +                                                   bool indirect, bool premapped)
>  {
>         u16 next;
>
> @@ -518,7 +520,7 @@ static inline unsigned int virtqueue_add_desc_split(struct virtqueue *vq,
>         desc[i].addr = cpu_to_virtio64(vq->vdev, addr);
>         desc[i].len = cpu_to_virtio32(vq->vdev, len);
>
> -       extra[i].addr = addr;
> +       extra[i].addr = premapped ? DMA_MAPPING_ERROR : addr;
>         extra[i].len = len;
>         extra[i].flags = flags;
>
> @@ -611,7 +613,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>                          */
>                         i = virtqueue_add_desc_split(_vq, desc, extra, i, addr, sg->length,
>                                                      VRING_DESC_F_NEXT,
> -                                                    indirect);
> +                                                    indirect, sg_is_premapped(sg));
>                 }
>         }
>         for (; n < (out_sgs + in_sgs); n++) {
> @@ -629,12 +631,12 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>                                                      sg->length,
>                                                      VRING_DESC_F_NEXT |
>                                                      VRING_DESC_F_WRITE,
> -                                                    indirect);
> +                                                    indirect, sg_is_premapped(sg));
>                 }
>         }
>         /* Last one doesn't continue. */
>         desc[prev].flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
> -       if (!indirect && vring_need_unmap_buffer(vq))
> +       if (!indirect && vring_need_unmap_buffer(vq, &extra[prev]))
>                 vq->split.desc_extra[prev & (vq->split.vring.num - 1)].flags &=
>                         ~VRING_DESC_F_NEXT;
>
> @@ -643,19 +645,15 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>                 dma_addr_t addr = vring_map_single(
>                         vq, desc, total_sg * sizeof(struct vring_desc),
>                         DMA_TO_DEVICE);
> -               if (vring_mapping_error(vq, addr)) {
> -                       if (vq->premapped)
> -                               goto free_indirect;
> -
> +               if (vring_mapping_error(vq, addr))
>                         goto unmap_release;
> -               }
>
>                 virtqueue_add_desc_split(_vq, vq->split.vring.desc,
>                                          vq->split.desc_extra,
>                                          head, addr,
>                                          total_sg * sizeof(struct vring_desc),
>                                          VRING_DESC_F_INDIRECT,
> -                                        false);
> +                                        false, false);
>         }
>
>         /* We're using some buffers from the free list. */
> @@ -712,7 +710,6 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>                 i = vring_unmap_one_split(vq, &extra[i]);
>         }
>
> -free_indirect:
>         if (indirect)
>                 kfree(desc);
>
> @@ -794,7 +791,7 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
>                                 VRING_DESC_F_INDIRECT));
>                 BUG_ON(len == 0 || len % sizeof(struct vring_desc));
>
> -               if (vring_need_unmap_buffer(vq)) {
> +               if (vq->use_dma_api) {
>                         for (j = 0; j < len / sizeof(struct vring_desc); j++)
>                                 vring_unmap_one_split(vq, &extra[j]);
>                 }
> @@ -1228,7 +1225,7 @@ static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
>                                  (flags & VRING_DESC_F_WRITE) ?
>                                  DMA_FROM_DEVICE : DMA_TO_DEVICE);
>         } else {
> -               if (!vring_need_unmap_buffer(vq))
> +               if (!vring_need_unmap_buffer(vq, extra))
>                         return;
>
>                 dma_unmap_page(vring_dma_dev(vq),
> @@ -1309,7 +1306,7 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
>                         i++;
>
>                         if (unlikely(vq->use_dma_api)) {
> -                               extra[i].addr = addr;
> +                               extra[i].addr = sg_is_premapped(sg) ? DMA_MAPPING_ERROR : addr;
>                                 extra[i].len = sg->length;
>                                 extra[i].flags = n < out_sgs ?  0 : VRING_DESC_F_WRITE;
>                         }
> @@ -1320,12 +1317,8 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
>         addr = vring_map_single(vq, desc,
>                         total_sg * sizeof(struct vring_packed_desc),
>                         DMA_TO_DEVICE);
> -       if (vring_mapping_error(vq, addr)) {
> -               if (vq->premapped)
> -                       goto free_desc;
> -
> +       if (vring_mapping_error(vq, addr))
>                 goto unmap_release;
> -       }
>
>         vq->packed.vring.desc[head].addr = cpu_to_le64(addr);
>         vq->packed.vring.desc[head].len = cpu_to_le32(total_sg *
> @@ -1383,7 +1376,6 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
>         for (i = 0; i < err_idx; i++)
>                 vring_unmap_extra_packed(vq, &extra[i]);
>
> -free_desc:
>         kfree(desc);
>
>         END_USE(vq);
> @@ -1474,7 +1466,8 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
>                         desc[i].id = cpu_to_le16(id);
>
>                         if (unlikely(vq->use_dma_api)) {
> -                               vq->packed.desc_extra[curr].addr = addr;
> +                               vq->packed.desc_extra[curr].addr = sg_is_premapped(sg) ?
> +                                       DMA_MAPPING_ERROR : addr;
>                                 vq->packed.desc_extra[curr].len = sg->length;
>                                 vq->packed.desc_extra[curr].flags =
>                                         le16_to_cpu(flags);
> @@ -1625,10 +1618,9 @@ static void detach_buf_packed(struct vring_virtqueue *vq,
>                 if (!extra)
>                         return;
>
> -               if (vring_need_unmap_buffer(vq)) {
> +               if (vq->use_dma_api) {
>                         len = vq->packed.desc_extra[id].len;
> -                       for (i = 0; i < len / sizeof(struct vring_packed_desc);
> -                                       i++)
> +                       for (i = 0; i < len / sizeof(struct vring_packed_desc); i++)

Unnecessary changes?


>                                 vring_unmap_extra_packed(vq, &extra[i]);
>                 }
>                 kfree(desc);
> @@ -2212,6 +2204,11 @@ static inline int virtqueue_add(struct virtqueue *_vq,
>   * @data: the token identifying the buffer.
>   * @gfp: how to do memory allocations (if necessary).
>   *
> + * When sg_page(sg) is NULL, this indicates that the driver has performed DMA
> + * mapping in advance, allowing the virtio core to directly utilize
> + * sg_dma_address(sg) without conducting any internal DMA mapping. Additionally,
> + * DMA unmap operations for this buffer will be bypassed.
> + *
>   * Caller must ensure we don't call this with other virtqueue operations
>   * at the same time (except where noted).
>   *
> @@ -2246,6 +2243,11 @@ EXPORT_SYMBOL_GPL(virtqueue_add_sgs);
>   * @data: the token identifying the buffer.
>   * @gfp: how to do memory allocations (if necessary).
>   *
> + * When sg_page(sg) is NULL, this indicates that the driver has performed DMA
> + * mapping in advance, allowing the virtio core to directly utilize
> + * sg_dma_address(sg) without conducting any internal DMA mapping. Additionally,
> + * DMA unmap operations for this buffer will be bypassed.
> + *
>   * Caller must ensure we don't call this with other virtqueue operations
>   * at the same time (except where noted).
>   *
> @@ -2268,6 +2270,11 @@ EXPORT_SYMBOL_GPL(virtqueue_add_outbuf);
>   * @data: the token identifying the buffer.
>   * @gfp: how to do memory allocations (if necessary).
>   *
> + * When sg_page(sg) is NULL, this indicates that the driver has performed DMA
> + * mapping in advance, allowing the virtio core to directly utilize
> + * sg_dma_address(sg) without conducting any internal DMA mapping. Additionally,
> + * DMA unmap operations for this buffer will be bypassed.
> + *
>   * Caller must ensure we don't call this with other virtqueue operations
>   * at the same time (except where noted).
>   *
> @@ -2291,6 +2298,11 @@ EXPORT_SYMBOL_GPL(virtqueue_add_inbuf);
>   * @ctx: extra context for the token
>   * @gfp: how to do memory allocations (if necessary).
>   *
> + * When sg_page(sg) is NULL, this indicates that the driver has performed DMA
> + * mapping in advance, allowing the virtio core to directly utilize
> + * sg_dma_address(sg) without conducting any internal DMA mapping. Additionally,
> + * DMA unmap operations for this buffer will be bypassed.
> + *
>   * Caller must ensure we don't call this with other virtqueue operations
>   * at the same time (except where noted).
>   *
> --
> 2.32.0.3.g01195cf9f
>

Thanks






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