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

[Andy Gospodarek <andrew.gospodarek@broadcom.com>]

On Thu, Jan 05, 2023 at 10:16:42AM -0800, 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?

100% 

Seems like we are back to speed vs ease of use, then?

> > 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 :)

I didn't recall that -- maybe I'll check the archives and see what I can
find.