Re: [PATCH iwl-next v3 16/18] idpf: add support for XDP on Rx

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

From: Alexander Lobakin <aleksander.lobakin@intel.com>
Date: Fri, 1 Aug 2025 15:12:43 +0200

> From: Kees Cook <kees@kernel.org>
> Date: Thu, 31 Jul 2025 10:05:47 -0700
> 
>> On Thu, Jul 31, 2025 at 01:37:34PM +0100, Simon Horman wrote:
>>> While I appreciate the desire for improved performance and nicer code
>>> generation. I think the idea of writing 64 bits of data to the
>>> address of a 32 bit member of a structure goes against the direction
>>> of hardening work by Kees and others.
>>
>> Agreed: it's better to avoid obscuring these details from the compiler
>> so it can have an "actual" view of the object sizes involved.
>>
>>> Indeed, it seems to me this is the kind of thing that struct_group()
>>> aims to avoid.
>>>
>>> In this case struct group() doesn't seem like the best option,
>>> because it would provide a 64-bit buffer that we can memcpy into.
>>> But it seems altogether better to simply assign u64 value to a u64 member.
>>
>> Agreed: with struct_group you get a sized pointer, and while you can
>> provide a struct tag to make it an assignable object, it doesn't make
>> too much sense here.
>>
>>> So I'm wondering if an approach along the following lines is appropriate
>>> (Very lightly compile tested only!).
>>>
>>> And yes, there is room for improvement of the wording of the comment
>>> I included below.
>>>
>>> diff --git a/include/net/libeth/xdp.h b/include/net/libeth/xdp.h
>>> index f4880b50e804..a7d3d8e44aa6 100644
>>> --- a/include/net/libeth/xdp.h
>>> +++ b/include/net/libeth/xdp.h
>>> @@ -1283,11 +1283,7 @@ static inline void libeth_xdp_prepare_buff(struct libeth_xdp_buff *xdp,
>>>  	const struct page *page = __netmem_to_page(fqe->netmem);
>>>  
>>>  #ifdef __LIBETH_WORD_ACCESS
>>> -	static_assert(offsetofend(typeof(xdp->base), flags) -
>>> -		      offsetof(typeof(xdp->base), frame_sz) ==
>>> -		      sizeof(u64));
>>> -
>>> -	*(u64 *)&xdp->base.frame_sz = fqe->truesize;
>>> +	xdp->base.frame_sz_le_qword = fqe->truesize;
>>>  #else
>>>  	xdp_init_buff(&xdp->base, fqe->truesize, xdp->base.rxq);
>>>  #endif
>>> diff --git a/include/net/xdp.h b/include/net/xdp.h
>>> index b40f1f96cb11..b5eedeb82c9b 100644
>>> --- a/include/net/xdp.h
>>> +++ b/include/net/xdp.h
>>> @@ -85,8 +85,19 @@ struct xdp_buff {
>>>  	void *data_hard_start;
>>>  	struct xdp_rxq_info *rxq;
>>>  	struct xdp_txq_info *txq;
>>> -	u32 frame_sz; /* frame size to deduce data_hard_end/reserved tailroom*/
>>> -	u32 flags; /* supported values defined in xdp_buff_flags */
>>> +	union {
>>> +		/* Allow setting frame_sz and flags as a single u64 on
>>> +		 * little endian systems. This may may give optimal
>>> +		 * performance. */
>>> +		u64 frame_sz_le_qword;
>>> +		struct {
>>> +			/* Frame size to deduce data_hard_end/reserved
>>> +			 * tailroom. */
>>> +			u32 frame_sz;
>>> +			/* Supported values defined in xdp_buff_flags. */
>>> +			u32 flags;
>>> +		};
>>> +	};
>>>  };
>>
>> Yeah, this looks like a nice way to express this, and is way more
>> descriptive than "(u64 *)&xdp->base.frame_sz" :)
> 
> Sounds good to me!
> 
> Let me send v4 where I'll fix this.

Note: would it be okay if I send v4 with this fix when the window opens,
while our validation will retest v3 from Tony's tree in meantine? It's a
cosmetic change anyway and does not involve any functional changes.

Thanks,
Olek