xdp: transition into using xdp_frame for return API

Changing API xdp_return_frame() to take struct xdp_frame as argument,
seems like a natural choice. But there are some subtle performance
details here that needs extra care, which is a deliberate choice.

When de-referencing xdp_frame on a remote CPU during DMA-TX
completion, result in the cache-line is change to "Shared"
state. Later when the page is reused for RX, then this xdp_frame
cache-line is written, which change the state to "Modified".

This situation already happens (naturally) for, virtio_net, tun and
cpumap as the xdp_frame pointer is the queued object.  In tun and
cpumap, the ptr_ring is used for efficiently transferring cache-lines
(with pointers) between CPUs. Thus, the only option is to
de-referencing xdp_frame.

It is only the ixgbe driver that had an optimization, in which it can
avoid doing the de-reference of xdp_frame.  The driver already have
TX-ring queue, which (in case of remote DMA-TX completion) have to be
transferred between CPUs anyhow.  In this data area, we stored a
struct xdp_mem_info and a data pointer, which allowed us to avoid
de-referencing xdp_frame.

To compensate for this, a prefetchw is used for telling the cache
coherency protocol about our access pattern.  My benchmarks show that
this prefetchw is enough to compensate the ixgbe driver.

V7: Adjust for commit d9314c47 ("i40e: add support for XDP_REDIRECT")
V8: Adjust for commit bd658dda ("net/mlx5e: Separate dma base address
and offset in dma_sync call")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/intel/i40e/i40e_txrx.c

@@ -638,8 +638,7 @@ static void i40e_unmap_and_free_tx_resource(struct i40e_ring *ring,
 		if (tx_buffer->tx_flags & I40E_TX_FLAGS_FD_SB)
 			kfree(tx_buffer->raw_buf);
 		else if (ring_is_xdp(ring))
-			xdp_return_frame(tx_buffer->xdpf->data,
-					 &tx_buffer->xdpf->mem);
+			xdp_return_frame(tx_buffer->xdpf);
 		else
 			dev_kfree_skb_any(tx_buffer->skb);
 		if (dma_unmap_len(tx_buffer, len))
@@ -842,7 +841,7 @@ static bool i40e_clean_tx_irq(struct i40e_vsi *vsi,
 
 		/* free the skb/XDP data */
 		if (ring_is_xdp(tx_ring))
-			xdp_return_frame(tx_buf->xdpf->data, &tx_buf->xdpf->mem);
+			xdp_return_frame(tx_buf->xdpf);
 		else
 			napi_consume_skb(tx_buf->skb, napi_budget);
 

drivers/net/ethernet/intel/ixgbe/ixgbe.h

@@ -241,8 +241,7 @@ struct ixgbe_tx_buffer {
 	unsigned long time_stamp;
 	union {
 		struct sk_buff *skb;
-		/* XDP uses address ptr on irq_clean */
-		void *data;
+		struct xdp_frame *xdpf;
 	};
 	unsigned int bytecount;
 	unsigned short gso_segs;
@@ -250,7 +249,6 @@ struct ixgbe_tx_buffer {
 	DEFINE_DMA_UNMAP_ADDR(dma);
 	DEFINE_DMA_UNMAP_LEN(len);
 	u32 tx_flags;
-	struct xdp_mem_info xdp_mem;
 };
 
 struct ixgbe_rx_buffer {

drivers/net/ethernet/intel/ixgbe/ixgbe_main.c

@@ -1216,7 +1216,7 @@ static bool ixgbe_clean_tx_irq(struct ixgbe_q_vector *q_vector,
 
 		/* free the skb */
 		if (ring_is_xdp(tx_ring))
-			xdp_return_frame(tx_buffer->data, &tx_buffer->xdp_mem);
+			xdp_return_frame(tx_buffer->xdpf);
 		else
 			napi_consume_skb(tx_buffer->skb, napi_budget);
 
@@ -2386,6 +2386,7 @@ static int ixgbe_clean_rx_irq(struct ixgbe_q_vector *q_vector,
 			xdp.data_hard_start = xdp.data -
 					      ixgbe_rx_offset(rx_ring);
 			xdp.data_end = xdp.data + size;
+			prefetchw(xdp.data_hard_start); /* xdp_frame write */
 
 			skb = ixgbe_run_xdp(adapter, rx_ring, &xdp);
 		}
@@ -5797,7 +5798,7 @@ static void ixgbe_clean_tx_ring(struct ixgbe_ring *tx_ring)
 
 		/* Free all the Tx ring sk_buffs */
 		if (ring_is_xdp(tx_ring))
-			xdp_return_frame(tx_buffer->data, &tx_buffer->xdp_mem);
+			xdp_return_frame(tx_buffer->xdpf);
 		else
 			dev_kfree_skb_any(tx_buffer->skb);
 
@@ -8348,16 +8349,21 @@ static int ixgbe_xmit_xdp_ring(struct ixgbe_adapter *adapter,
 	struct ixgbe_ring *ring = adapter->xdp_ring[smp_processor_id()];
 	struct ixgbe_tx_buffer *tx_buffer;
 	union ixgbe_adv_tx_desc *tx_desc;
+	struct xdp_frame *xdpf;
 	u32 len, cmd_type;
 	dma_addr_t dma;
 	u16 i;
 
-	len = xdp->data_end - xdp->data;
+	xdpf = convert_to_xdp_frame(xdp);
+	if (unlikely(!xdpf))
+		return -EOVERFLOW;
+
+	len = xdpf->len;
 
 	if (unlikely(!ixgbe_desc_unused(ring)))
 		return IXGBE_XDP_CONSUMED;
 
-	dma = dma_map_single(ring->dev, xdp->data, len, DMA_TO_DEVICE);
+	dma = dma_map_single(ring->dev, xdpf->data, len, DMA_TO_DEVICE);
 	if (dma_mapping_error(ring->dev, dma))
 		return IXGBE_XDP_CONSUMED;
 
@@ -8372,8 +8378,7 @@ static int ixgbe_xmit_xdp_ring(struct ixgbe_adapter *adapter,
 
 	dma_unmap_len_set(tx_buffer, len, len);
 	dma_unmap_addr_set(tx_buffer, dma, dma);
-	tx_buffer->data = xdp->data;
-	tx_buffer->xdp_mem = xdp->rxq->mem;
+	tx_buffer->xdpf = xdpf;
 
 	tx_desc->read.buffer_addr = cpu_to_le64(dma);
 

drivers/net/ethernet/mellanox/mlx5/core/en_rx.c

@@ -890,6 +890,7 @@ struct sk_buff *skb_from_cqe(struct mlx5e_rq *rq, struct mlx5_cqe64 *cqe,
 
 	dma_sync_single_range_for_cpu(rq->pdev, di->addr, wi->offset,
 				      frag_size, DMA_FROM_DEVICE);
+	prefetchw(va); /* xdp_frame data area */
 	prefetch(data);
 	wi->offset += frag_size;
 

drivers/net/tun.c

@@ -663,7 +663,7 @@ void tun_ptr_free(void *ptr)
 	if (tun_is_xdp_frame(ptr)) {
 		struct xdp_frame *xdpf = tun_ptr_to_xdp(ptr);
 
-		xdp_return_frame(xdpf->data, &xdpf->mem);
+		xdp_return_frame(xdpf);
 	} else {
 		__skb_array_destroy_skb(ptr);
 	}
@@ -2196,7 +2196,7 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
 		struct xdp_frame *xdpf = tun_ptr_to_xdp(ptr);
 
 		ret = tun_put_user_xdp(tun, tfile, xdpf, to);
-		xdp_return_frame(xdpf->data, &xdpf->mem);
+		xdp_return_frame(xdpf);
 	} else {
 		struct sk_buff *skb = ptr;
 

drivers/net/virtio_net.c

@@ -430,7 +430,7 @@ static int __virtnet_xdp_xmit(struct virtnet_info *vi,
 
 	/* Free up any pending old buffers before queueing new ones. */
 	while ((xdpf_sent = virtqueue_get_buf(sq->vq, &len)) != NULL)
-		xdp_return_frame(xdpf_sent->data, &xdpf_sent->mem);
+		xdp_return_frame(xdpf_sent);
 
 	xdpf = convert_to_xdp_frame(xdp);
 	if (unlikely(!xdpf))

include/net/xdp.h

@@ -103,7 +103,7 @@ struct xdp_frame *convert_to_xdp_frame(struct xdp_buff *xdp)
 	return xdp_frame;
 }
 
-void xdp_return_frame(void *data, struct xdp_mem_info *mem);
+void xdp_return_frame(struct xdp_frame *xdpf);
 
 int xdp_rxq_info_reg(struct xdp_rxq_info *xdp_rxq,
 		     struct net_device *dev, u32 queue_index);

kernel/bpf/cpumap.c

@@ -219,7 +219,7 @@ static void __cpu_map_ring_cleanup(struct ptr_ring *ring)
 
 	while ((xdpf = ptr_ring_consume(ring)))
 		if (WARN_ON_ONCE(xdpf))
-			xdp_return_frame(xdpf->data, &xdpf->mem);
+			xdp_return_frame(xdpf);
 }
 
 static void put_cpu_map_entry(struct bpf_cpu_map_entry *rcpu)
@@ -275,7 +275,7 @@ static int cpu_map_kthread_run(void *data)
 
 			skb = cpu_map_build_skb(rcpu, xdpf);
 			if (!skb) {
-				xdp_return_frame(xdpf->data, &xdpf->mem);
+				xdp_return_frame(xdpf);
 				continue;
 			}
 
@@ -578,7 +578,7 @@ static int bq_flush_to_queue(struct bpf_cpu_map_entry *rcpu,
 		err = __ptr_ring_produce(q, xdpf);
 		if (err) {
 			drops++;
-			xdp_return_frame(xdpf->data, &xdpf->mem);
+			xdp_return_frame(xdpf);
 		}
 		processed++;
 	}

net/core/xdp.c

@@ -308,9 +308,11 @@ int xdp_rxq_info_reg_mem_model(struct xdp_rxq_info *xdp_rxq,
 }
 EXPORT_SYMBOL_GPL(xdp_rxq_info_reg_mem_model);
 
-void xdp_return_frame(void *data, struct xdp_mem_info *mem)
+void xdp_return_frame(struct xdp_frame *xdpf)
 {
+	struct xdp_mem_info *mem = &xdpf->mem;
 	struct xdp_mem_allocator *xa;
+	void *data = xdpf->data;
 	struct page *page;
 
 	switch (mem->type) {