Re: [RFC PATCHv2 1/1] idpf: Fix header clobber in IDPF with SWIOTLB enabled

[Alexander Lobakin <aleksander.lobakin@intel.com>]

Hey,

From: Steve Rutherford via Intel-wired-lan <intel-wired-lan@osuosl.org>
Date: Fri, 6 Mar 2026 11:35:27 -0800

> On Fri, Mar 6, 2026 at 6:52=E2=80=AFAM Alexander Lobakin
> <aleksander.lobakin@intel.com> wrote:
> >
> > From: Steve Rutherford <srutherford@google.com>
> > Date: Wed, 4 Mar 2026 14:01:46 -0800
> >
> > > I believe syncing twice isn't inherently wrong - it's more that you
> > > can't synthesize the header via the workaround and then sync, since it
> > > will pull the uninitialized header buffer from the SWIOTLB. Outside of
> > > SWIOTLB, dma syncs are more or less no-ops, while (with SWIOTLB) they
> > > are copies from/to the bounce buffers.
> >
> > Ah I see.
> >
> > What if I add sync_for_device after copying the header? This should
> > synchronize the bounce buffer with the copied data I guess? A bit of
> > overhead, but this W/A triggers mostly on stuff like ARP/ICMP, "hotpath"
> > L4 protos are fortunately not affected.
> 
> That should work fine as well. I'm not certain I have strong
> preferences on the right answer here, other than "does it work and,
> ideally, is it less confusing?" The patch I posted is a bit
> unintuitive. I think what you are describing might make the workaround
> self-contained.

Could you please test this patch with SWIOTLB? If it doesn't fix
the issue, you can try changing `page_pool_get_dma_dir(hdr_pp)`
to `DMA_TO_DEVICE` and/or `DMA_BIDIRECTIONAL`.
Currently, I don't have any machines with SWIOTLB unfortunately =\
Let me know if any of these works. I'll submit it properly when we
have a solution.

(the patch applies cleanly to the latest net-next and should apply
 to a couple older kernel releases as well)

> 
> thanks,
> Steve
>  [And sorry for my gmail-driven top posting crimes D: ]

Thanks,
Olek
---
diff --git a/drivers/net/ethernet/intel/idpf/idpf_txrx.c b/drivers/net/ethernet/intel/idpf/idpf_txrx.c
index 45ee5b80479a..42111d56d66f 100644
--- a/drivers/net/ethernet/intel/idpf/idpf_txrx.c
+++ b/drivers/net/ethernet/intel/idpf/idpf_txrx.c
@@ -3475,7 +3475,8 @@ static u32 idpf_rx_hsplit_wa(const struct libeth_fqe *hdr,
 			     struct libeth_fqe *buf, u32 data_len)
 {
 	u32 copy = data_len <= L1_CACHE_BYTES ? data_len : ETH_HLEN;
-	struct page *hdr_page, *buf_page;
+	const struct page_pool *hdr_pp;
+	dma_addr_t hdr_addr;
 	const void *src;
 	void *dst;
 
@@ -3483,16 +3484,20 @@ static u32 idpf_rx_hsplit_wa(const struct libeth_fqe *hdr,
 	    !libeth_rx_sync_for_cpu(buf, copy))
 		return 0;
 
-	hdr_page = __netmem_to_page(hdr->netmem);
-	buf_page = __netmem_to_page(buf->netmem);
-	dst = page_address(hdr_page) + hdr->offset +
-		pp_page_to_nmdesc(hdr_page)->pp->p.offset;
-	src = page_address(buf_page) + buf->offset +
-		pp_page_to_nmdesc(buf_page)->pp->p.offset;
+	hdr_pp = __netmem_get_pp(hdr->netmem);
+	dst = __netmem_address(hdr->netmem) + hdr->offset + hdr_pp->p.offset;
+	src = __netmem_address(buf->netmem) + buf->offset +
+	      __netmem_get_pp(buf->netmem)->p.offset;
 
 	memcpy(dst, src, LARGEST_ALIGN(copy));
 	buf->offset += copy;
 
+	/* Make sure SWIOTLB is synced */
+	hdr_addr = page_pool_get_dma_addr_netmem(hdr->netmem);
+	dma_sync_single_range_for_device(hdr_pp->p.dev, hdr_addr,
+					 hdr->offset + hdr_pp->p.offset,
+					 copy, page_pool_get_dma_dir(hdr_pp));
+
 	return copy;
 }