Re: [RFC bpf-next v1 0/7] Add kfunc bpf_xdp_pull_data

[Martin KaFai Lau <martin.lau@linux.dev>]

On 9/4/25 10:28 AM, Amery Hung wrote:
> On Fri, Aug 29, 2025 at 11:22 AM Martin KaFai Lau <martin.lau@linux.dev> wrote:
>>
>> On 8/28/25 6:39 AM, Nimrod Oren wrote:
>>> I'm currently working on a series that converts the xdp_native program
>>> to use dynptr for accessing header data. If accepted, it should provide
>>> better performance, since dynptr can access without copying the data.
>>
>> The bpf_xdp_adjust_tail is aware of xdp_buff_has_frags. Is there a reason that
>> bpf_xdp_adjust_head cannot handle frags also?
> 
> I am not aware of reasons that would stop this.
> 
> Are you suggesting another way to pop headers? E.g., use
> bpf_xdp_adjust_head() to shrink the first frag from the front and call
> bpf_xdp_store_bytes() to move the remaining headers

bpf_xdp_pull_data is useful on its own, nothing change there. On top of that, 
bpf_xdp_adjust_head() should be useful also for bpf prog that does not care 
about the linear/frag layout and stay with the bpf_dynptr helpers (or the 
bpf_xdp_{store,load}_bytes you mentioned). Also, if I read it correctly, 
bpf_xdp_adjust_head can increase the head of multi buf xdp but not shrinking it 
while the bpf_xdp_adjust_tail can do both. It could be a surprise to use. The 
adjust_head support can be a followup though. I think some of your work in this 
series is pretty close to having adjust_head support also, so I was wondering if 
there is reason that adjust_head cannot be supported.