Re: [PATCH bpf-next 00/16] bpf: Introduce BPF arena.

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

On Wed, Feb 7, 2024 at 5:33 AM Barret Rhoden <brho@google.com> wrote:
>
>
> if i knew userspace wasn't going to follow pointers, NO_USER_CONV would
> both be a speedup and make it so i don't have to worry about mmapping to
> the same virtual address in every process that shares the arena map.
> though this latter feature isn't in the code.  right now you have to
> have it mmapped at the same user_va in all address spaces.  that's not a
> huge deal for me either way.

Not quite. With:

struct {
   __uint(type, BPF_MAP_TYPE_ARENA);
...
   __ulong(map_extra, 2ull << 44); /* start of mmap() region */
...
} arena SEC(".maps");

the future user_vm_start will be specified at map creation time,
so arena doesn't have to be mapped to be usable.
But having an address known early helps potential future mmap-s.
Like to examine the state of the arena with bpftool.