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 d9314c474d4f ("i40e: add support for XDP_REDIRECT")
V8: Adjust for commit bd658dda4237 ("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>

1 files changed, 2 insertions, 3 deletions

diff --git a/drivers/net/ethernet/intel/i40e/i40e_txrx.c b/drivers/net/ethernet/intel/i40e/i40e_txrx.c
index 96c54cbfb1f9..c8bf4d35fdea 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_txrx.c
+++ b/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);