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, 2 deletions

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 283bde85c455..bec130cdbd9d 100644
--- a/drivers/net/tun.c
+++ b/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;