Re: [PATCH net-next v2 04/13] virtio_ring: perform premapped operations based on per-buffer

Jason Wang
2024-11-05T04:25:24Z

On Wed, Oct 30, 2024 at 4:25 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> 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,

Even worse as MAX_SKB_FRAGS could be even larger.

> 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.
>
> And consider that some sgs are not generated by the virtio driver,
> that may be passed from the block stack. So we can not change the
> sgs, new APIs are the better way.

I had some new thoughts on this.

Pre-mapping makes a lot of sense as it allows the us to move the
expensive DMA mapping operations (swiotlb, IOMMU or VDUSE) out of the
per-vq lock. I wonder what would we do if someday we want to convert
the skb TX to be premapped (or even all virtio drivers).

Considering we've already used skb_to_sgvec() in start_xmit() it looks
like we need to allocate sg[queue_num][MAX_SKB_FRAGS + 2] and store
the dma mapping there. Then we probably don't even need to duplicate
the dma mapping information in the core.

It means it's the driver's charge to store the dma information via sg,
so we can simply use dma_map_sg() in add_sgs() which allows various
optimizations in IOMMU layers.

>
> So we pass the new argument 'premapped' to indicate the buffers
> submitted to virtio are premapped in advance. Additionally,
> DMA unmap operations for these buffers will be bypassed.
>
> Suggested-by: Jason Wang <jasowang@redhat.com>
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>

Anyhow

Acked-by: Jason Wang <jasowang@redhat.com>

Thanks

> ---
> drivers/virtio/virtio_ring.c | 79 ++++++++++++++++++------------------
> 1 file changed, 40 insertions(+), 39 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 628e01af1c9a..a89295b79e66 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -300,9 +300,10 @@ 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)
> @@ -372,9 +373,10 @@ static struct device *vring_dma_dev(const struct vring_virtqueue *vq)
>
> /* Map one sg entry. */
> static int vring_map_one_sg(const struct vring_virtqueue *vq, struct scatterlist *sg,
> - enum dma_data_direction direction, dma_addr_t *addr)
> + enum dma_data_direction direction, dma_addr_t *addr,
> + bool premapped)
> {
> - if (vq->premapped) {
> + if (premapped) {
> *addr = sg_dma_address(sg);
> return 0;
> }
> @@ -465,7 +467,7 @@ 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),
> @@ -514,7 +516,7 @@ static inline unsigned int virtqueue_add_desc_split(struct virtqueue *vq,
> unsigned int i,
> dma_addr_t addr,
> unsigned int len,
> - u16 flags)
> + u16 flags, bool premapped)
> {
> u16 next;
>
> @@ -522,7 +524,7 @@ 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;
>
> @@ -540,6 +542,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> unsigned int in_sgs,
> void *data,
> void *ctx,
> + bool premapped,
> gfp_t gfp)
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
> @@ -606,7 +609,7 @@ 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))
> + if (vring_map_one_sg(vq, sg, DMA_TO_DEVICE, &addr, premapped))
> goto unmap_release;
>
> prev = i;
> @@ -614,14 +617,15 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> * table since it use stream DMA mapping.
> */
> i = virtqueue_add_desc_split(_vq, desc, extra, i, addr, sg->length,
> - VRING_DESC_F_NEXT);
> + VRING_DESC_F_NEXT,
> + premapped);
> }
> }
> for (; n < (out_sgs + in_sgs); n++) {
> for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> dma_addr_t addr;
>
> - if (vring_map_one_sg(vq, sg, DMA_FROM_DEVICE, &addr))
> + if (vring_map_one_sg(vq, sg, DMA_FROM_DEVICE, &addr, premapped))
> goto unmap_release;
>
> prev = i;
> @@ -631,12 +635,13 @@ 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 |
> - VRING_DESC_F_WRITE);
> + VRING_DESC_F_WRITE,
> + premapped);
> }
> }
> /* 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;
>
> @@ -645,18 +650,14 @@ 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);
> + VRING_DESC_F_INDIRECT, false);
> }
>
> /* We're using some buffers from the free list. */
> @@ -713,7 +714,6 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> i = vring_unmap_one_split(vq, &extra[i]);
> }
>
> -free_indirect:
> if (indirect)
> kfree(desc);
>
> @@ -798,7 +798,7 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
>
> extra = (struct vring_desc_extra *)&indir_desc[num];
>
> - if (vring_need_unmap_buffer(vq)) {
> + if (vq->use_dma_api) {
> for (j = 0; j < num; j++)
> vring_unmap_one_split(vq, &extra[j]);
> }
> @@ -1232,7 +1232,7 @@ 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),
> @@ -1276,6 +1276,7 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> unsigned int out_sgs,
> unsigned int in_sgs,
> void *data,
> + bool premapped,
> gfp_t gfp)
> {
> struct vring_desc_extra *extra;
> @@ -1306,7 +1307,8 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> for (n = 0; n < out_sgs + in_sgs; n++) {
> for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> if (vring_map_one_sg(vq, sg, n < out_sgs ?
> - DMA_TO_DEVICE : DMA_FROM_DEVICE, &addr))
> + DMA_TO_DEVICE : DMA_FROM_DEVICE,
> + &addr, premapped))
> goto unmap_release;
>
> desc[i].flags = cpu_to_le16(n < out_sgs ?
> @@ -1315,7 +1317,7 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> desc[i].len = cpu_to_le32(sg->length);
>
> if (unlikely(vq->use_dma_api)) {
> - extra[i].addr = addr;
> + extra[i].addr = premapped ? DMA_MAPPING_ERROR : addr;
> extra[i].len = sg->length;
> extra[i].flags = n < out_sgs ? 0 : VRING_DESC_F_WRITE;
> }
> @@ -1328,12 +1330,8 @@ 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 *
> @@ -1391,7 +1389,6 @@ 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);
> @@ -1405,6 +1402,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> unsigned int in_sgs,
> void *data,
> void *ctx,
> + bool premapped,
> gfp_t gfp)
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
> @@ -1431,7 +1429,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
>
> if (virtqueue_use_indirect(vq, total_sg)) {
> err = virtqueue_add_indirect_packed(vq, sgs, total_sg, out_sgs,
> - in_sgs, data, gfp);
> + in_sgs, data, premapped, gfp);
> if (err != -ENOMEM) {
> END_USE(vq);
> return err;
> @@ -1466,7 +1464,8 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> dma_addr_t addr;
>
> if (vring_map_one_sg(vq, sg, n < out_sgs ?
> - DMA_TO_DEVICE : DMA_FROM_DEVICE, &addr))
> + DMA_TO_DEVICE : DMA_FROM_DEVICE,
> + &addr, premapped))
> goto unmap_release;
>
> flags = cpu_to_le16(vq->packed.avail_used_flags |
> @@ -1482,7 +1481,8 @@ 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 = premapped ?
> + DMA_MAPPING_ERROR : addr;
> vq->packed.desc_extra[curr].len = sg->length;
> vq->packed.desc_extra[curr].flags =
> le16_to_cpu(flags);
> @@ -1633,7 +1633,7 @@ static void detach_buf_packed(struct vring_virtqueue *vq,
> if (!desc)
> return;
>
> - if (vring_need_unmap_buffer(vq)) {
> + if (vq->use_dma_api) {
> len = vq->packed.desc_extra[id].len;
> num = len / sizeof(struct vring_packed_desc);
>
> @@ -2204,14 +2204,15 @@ static inline int virtqueue_add(struct virtqueue *_vq,
> unsigned int in_sgs,
> void *data,
> void *ctx,
> + bool premapped,
> gfp_t gfp)
> {
> struct vring_virtqueue *vq = to_vvq(_vq);
>
> return vq->packed_ring ? virtqueue_add_packed(_vq, sgs, total_sg,
> - out_sgs, in_sgs, data, ctx, gfp) :
> + out_sgs, in_sgs, data, ctx, premapped, gfp) :
> virtqueue_add_split(_vq, sgs, total_sg,
> - out_sgs, in_sgs, data, ctx, gfp);
> + out_sgs, in_sgs, data, ctx, premapped, gfp);
> }
>
> /**
> @@ -2245,7 +2246,7 @@ int virtqueue_add_sgs(struct virtqueue *_vq,
> total_sg++;
> }
> return virtqueue_add(_vq, sgs, total_sg, out_sgs, in_sgs,
> - data, NULL, gfp);
> + data, NULL, false, gfp);
> }
> EXPORT_SYMBOL_GPL(virtqueue_add_sgs);
>
> @@ -2267,7 +2268,7 @@ int virtqueue_add_outbuf(struct virtqueue *vq,
> void *data,
> gfp_t gfp)
> {
> - return virtqueue_add(vq, &sg, num, 1, 0, data, NULL, gfp);
> + return virtqueue_add(vq, &sg, num, 1, 0, data, NULL, false, gfp);
> }
> EXPORT_SYMBOL_GPL(virtqueue_add_outbuf);
>
> @@ -2289,7 +2290,7 @@ int virtqueue_add_inbuf(struct virtqueue *vq,
> void *data,
> gfp_t gfp)
> {
> - return virtqueue_add(vq, &sg, num, 0, 1, data, NULL, gfp);
> + return virtqueue_add(vq, &sg, num, 0, 1, data, NULL, false, gfp);
> }
> EXPORT_SYMBOL_GPL(virtqueue_add_inbuf);
>
> @@ -2313,7 +2314,7 @@ int virtqueue_add_inbuf_ctx(struct virtqueue *vq,
> void *ctx,
> gfp_t gfp)
> {
> - return virtqueue_add(vq, &sg, num, 0, 1, data, ctx, gfp);
> + return virtqueue_add(vq, &sg, num, 0, 1, data, ctx, false, gfp);
> }
> EXPORT_SYMBOL_GPL(virtqueue_add_inbuf_ctx);
>
> --
> 2.32.0.3.g01195cf9f
>