Re: [PATCH v6 bpf-next 6/7] bpf: introduce bpf_prog_pack allocator

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Fri, Jan 21, 2022 at 5:30 PM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Jan 21, 2022, at 5:12 PM, Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> >
> > On Fri, Jan 21, 2022 at 5:01 PM Song Liu <songliubraving@fb.com> wrote:
> >>
> >> In this way, we need to allocate rw_image here, and free it in
> >> bpf_jit_comp.c. This feels a little weird to me, but I guess that
> >> is still the cleanest solution for now.
> >
> > You mean inside bpf_jit_binary_alloc?
> > That won't be arch independent.
> > It needs to be split into generic piece that stays in core.c
> > and callbacks like bpf_jit_fill_hole_t
> > or into multiple helpers with prep in-between.
> > Don't worry if all archs need to be touched.
>
> How about we introduce callback bpf_jit_set_header_size_t? Then we
> can split x86's jit_fill_hole() into two functions, one to fill the
> hole, the other to set size. The rest of the logic gonna stay the same.
>
> Archs that do not use bpf_prog_pack won't need bpf_jit_set_header_size_t.

That's not any better.

Currently the choice of bpf_jit_binary_alloc_pack vs bpf_jit_binary_alloc
leaks into arch bits and bpf_prog_pack_max_size() doesn't
really make it generic.

Ideally all archs continue to use bpf_jit_binary_alloc()
and magic happens in a generic code.
If not then please remove bpf_prog_pack_max_size(),
since it doesn't provide much value and pick
bpf_jit_binary_alloc_pack() signature to fit x86 jit better.
It wouldn't need bpf_jit_fill_hole_t callback at all.
Please think it through so we don't need to redesign it
when another arch will decide to use huge pages for bpf progs.

cc-ing Ilya for ideas on how that would fit s390.