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

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

From: Alexander Lobakin <aleksander.lobakin@intel.com>
Date: Thu, 12 Mar 2026 17:30:24 +0100

> 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.

Any updates? I need your Tested-by in order to send this.

> 
> (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;
>  }

Thanks,
Olek