[PATCH net-next 5/5] virtio_net: Implement DMA pre-handler

From: Liang Chen
Date: Fri May 26 2023 - 01:47:33 EST


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;
+}
+
+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);
+ 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