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

From: Jakub Kicinski <[email-masked]>
Date: Thu, 5 Jan 2023 10:16:42 -0800

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 :)