On Fri, May 26, 2023 at 1:47 PM Liang Chen <liangchen.linux@xxxxxxxxx> wrote:
>
> Adding a DMA pre-handler that utilizes page pool for managing DMA mappings.
> When IOMMU is enabled, turning on the page_pool_dma_map module parameter to
> select page pool for DMA mapping management gives a significant reduction
> in the overhead caused by DMA mappings.
>
> In testing environments with a single core vm and qemu emulated IOMMU,
> significant performance improvements can be observed:
> Upstream codebase: 1.76 Gbits/sec
> Upstream codebase with page pool fragmentation support: 1.81 Gbits/sec
> Upstream codebase with page pool fragmentation and DMA support: 19.3
> Gbits/sec
>
> Signed-off-by: Liang Chen <liangchen.linux@xxxxxxxxx>
> ---
> drivers/net/virtio_net.c | 55 ++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 55 insertions(+)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index ac40b8c66c59..73cc4f9fe4fa 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -22,6 +22,7 @@
> #include <net/route.h>
> #include <net/xdp.h>
> #include <net/net_failover.h>
> +#include <linux/iommu.h>
>
> static int napi_weight = NAPI_POLL_WEIGHT;
> module_param(napi_weight, int, 0444);
> @@ -33,8 +34,10 @@ module_param(napi_tx, bool, 0644);
>
> static bool page_pool_enabled;
> static bool page_pool_frag;
> +static bool page_pool_dma_map;
> module_param(page_pool_enabled, bool, 0400);
> module_param(page_pool_frag, bool, 0400);
> +module_param(page_pool_dma_map, bool, 0400);
>
> /* FIXME: MTU in config. */
> #define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
> @@ -3830,6 +3833,49 @@ static void virtnet_del_vqs(struct virtnet_info *vi)
> virtnet_free_queues(vi);
> }
>
> +static dma_addr_t virtnet_pp_dma_map_page(struct device *dev, struct page *page,
> + unsigned long offset, size_t size,
> + enum dma_data_direction dir, unsigned long attrs)
> +{
> + struct page *head_page;
> +
> + if (dir != DMA_FROM_DEVICE)
> + return 0;
> +
> + head_page = compound_head(page);
> + return page_pool_get_dma_addr(head_page)
> + + (page - head_page) * PAGE_SIZE
> + + offset;
So it's not a map, it is just a query from the dma address from the pool.
> +}
> +
> +static bool virtnet_pp_dma_unmap_page(struct device *dev, dma_addr_t dma_handle,
> + size_t size, enum dma_data_direction dir,
> + unsigned long attrs)
> +{
> + phys_addr_t phys;
> +
> + /* Handle only the RX direction, and sync the DMA memory only if it's not
> + * a DMA coherent architecture.
> + */
> + if (dir != DMA_FROM_DEVICE)
> + return false;
> +
> + if (dev_is_dma_coherent(dev))
> + return true;
> +
> + phys = iommu_iova_to_phys(iommu_get_dma_domain(dev), dma_handle);
This would be somehow slow. If we track the mapping by driver, it
would be much faster.
More could be seen here:
https://lists.linuxfoundation.org/pipermail/virtualization/2023-May/066778.html
Thanks
> + if (WARN_ON(!phys))
> + return false;
> +
> + arch_sync_dma_for_cpu(phys, size, dir);
> + return true;
> +}
> +
> +static struct virtqueue_pre_dma_ops virtnet_pp_pre_dma_ops = {
> + .map_page = virtnet_pp_dma_map_page,
> + .unmap_page = virtnet_pp_dma_unmap_page,
> +};
> +
> static void virtnet_alloc_page_pool(struct receive_queue *rq)
> {
> struct virtio_device *vdev = rq->vq->vdev;
> @@ -3845,6 +3891,15 @@ static void virtnet_alloc_page_pool(struct receive_queue *rq)
> if (page_pool_frag)
> pp_params.flags |= PP_FLAG_PAGE_FRAG;
>
> + /* Consider using page pool DMA support only when DMA API is used. */
> + if (virtio_has_feature(vdev, VIRTIO_F_ACCESS_PLATFORM) &&
> + page_pool_dma_map) {
> + pp_params.flags |= PP_FLAG_DMA_MAP | PP_FLAG_DMA_SYNC_DEV;
> + pp_params.dma_dir = DMA_FROM_DEVICE;
> + pp_params.max_len = PAGE_SIZE << pp_params.order;
> + virtqueue_register_pre_dma_ops(rq->vq, &virtnet_pp_pre_dma_ops);
> + }
> +
> rq->page_pool = page_pool_create(&pp_params);
> if (IS_ERR(rq->page_pool)) {
> dev_warn(&vdev->dev, "page pool creation failed: %ld\n",
> --
> 2.31.1
>
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