Commit 03993094 authored by Jesper Dangaard Brouer's avatar Jesper Dangaard Brouer Committed by David S. Miller
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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: default avatarJesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 60bbf7ee

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

+2 −3
Original line number Diff line number Diff line
@@ -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

+1 −3
Original line number Diff line number Diff line
@@ -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

+11 −6
Original line number Diff line number Diff line
@@ -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

+1 −0
Original line number Diff line number Diff line
@@ -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

+2 −2
Original line number Diff line number Diff line
@@ -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;

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