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
