Re: [PATCH net-next 1/2] net: stmmac: fix XDP TX DMA address for compound RX pages

Hi Lorenzo,

Hi Daniel,

quoted

When RX buffers are compound pages (order > 0), only the head page
carries the DMA address programmed by the page pool
(page_pool_get_dma_addr()). stmmac_xdp_xmit_xdpf() computed the head
buffer's bus address from the page that happens to contain the data:

	page = frag ? skb_frag_page(frag)
		    : virt_to_page(xdpf->data);
	dma_addr = page_pool_get_dma_addr(page);
	...

RX pages become compound as soon as dma_buf_sz needs more than one page
(pp_params.order = order_base_2(num_pages) > 0), which happens from an
MTU around 3000 bytes once dma_buf_sz reaches 4096. If an XDP program
moves the data forward with bpf_xdp_adjust_head() by more than about a
page, xdpf->data points into a tail page whose struct page has no DMA
address: page_pool_get_dma_addr() then returns a bogus bus address, and
dma_sync_single_for_device() together with the TX descriptor transmit
from the wrong location.

Looks to me like mtu > ETH_DATA_LAN is rejected when setting a prog, and also
on mtu change when a prog is attached. So can this even happen today?

yes, these issues are real but at the moment we can't trigger them since we
have the MTU limitation in stmmac_change_mtu()/stmmac_xdp_set_prog().
These are preliminary fixes in order to remove this limitation.
There is a leftover issue we still need to fix about jumbo frames in
stmmac_xdp_xmit_xdpf().

Regards,
Lorenzo

quoted

Use virt_to_head_page() to read the DMA base of the buffer and keep the
offset relative to the head page (sizeof(*xdpf) + xdpf->headroom), which
is correct no matter which page the data ends up in.

I think the fix looks correct.

quoted

Fixes: be8b38a722e6 ("net: stmmac: Add support for XDP_TX action")
Signed-off-by: Lorenzo Bianconi <redacted>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index b8b4de4c5b1a..ca61ab449e32 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -5301,7 +5301,7 @@ static int stmmac_xdp_xmit_xdpf(struct stmmac_priv *priv, int queue,
 			struct page *page;
 
 			page = frag ? skb_frag_page(frag)
-				    : virt_to_page(xdpf->data);
+				    : virt_to_head_page(xdpf->data);
 			dma_addr = page_pool_get_dma_addr(page);
 			if (frag)
 				dma_addr += skb_frag_off(frag);

-- 
2.56.0

/Daniel