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, 3 insertions, 3 deletions

diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
index bcdc4dea5ce7..c95b04ec103e 100644
--- a/kernel/bpf/cpumap.c
+++ b/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++;
 	}