Re: [PATCH net-next v2] samples/bpf: fixup some tools to be able to support xdp multibuffer

[Tariq Toukan <ttoukan.linux@gmail.com>]

On 05/01/2023 20:16, Jakub Kicinski wrote:
> On Thu, 5 Jan 2023 11:57:32 -0500 Andy Gospodarek wrote:
>>> So my main concern would be that if we "allow" this, the only way to
>>> write an interoperable XDP program will be to use bpf_xdp_load_bytes()
>>> for every packet access. Which will be slower than DPA, so we may end up
>>> inadvertently slowing down all of the XDP ecosystem, because no one is
>>> going to bother with writing two versions of their programs. Whereas if
>>> you can rely on packet headers always being in the linear part, you can
>>> write a lot of the "look at headers and make a decision" type programs
>>> using just DPA, and they'll work for multibuf as well.
>>
>> The question I would have is what is really the 'slow down' for
>> bpf_xdp_load_bytes() vs DPA?  I know you and Jesper can tell me how many
>> instructions each use. :)
> 
> Until we have an efficient and inlined DPA access to frags an
> unconditional memcpy() of the first 2 cachelines-worth of headers
> in the driver must be faster than a piece-by-piece bpf_xdp_load_bytes()
> onto the stack, right?
> 
>> Taking a step back...years ago Dave mentioned wanting to make XDP
>> programs easy to write and it feels like using these accessor APIs would
>> help accomplish that.  If the kernel examples use bpf_xdp_load_bytes()
>> accessors everywhere then that would accomplish that.
> 
> I've been pushing for an skb_header_pointer()-like helper but
> the semantics were not universally loved :)

Maybe it's time to re-consider.

Is it something like an API that given an offset returns a pointer + 
allowed length to be accessed?

This sounds like a good direction to me, that avoids having any 
linear-part-length assumptions, while preserving good performance.

Maybe we can still require/guarantee that each single header (eth, ip, 
tcp, ...) does not cross a frag/page boundary. For otherwise, a prog 
needs to handle cases where headers span several fragments, so it has to 
reconstruct the header by copying the different parts into some local 
buffer.

This can be achieved by having another assumption that AFAIK already 
holds today: all fragments are of size PAGE_SIZE.

Regards,
Tariq