On Mon, Jul 18, 2022 at 12:07 PM Stanislav Fomichev <sdf@xxxxxxxxxx> wrote:
>
> Motivation:
>
> Our bpf programs have a bunch of options which are set at the loading
> time. After loading, they don't change. We currently use array map
> to store them and bpf program does the following:
>
> val = bpf_map_lookup_elem(&config_map, &key);
> if (likely(val && *val)) {
> // do some optional feature
> }
>
> Since the configuration is static and we have a lot of those features,
> I feel like we're wasting precious cycles doing dynamic lookups
> (and stalling on memory loads).
>
> I was assuming that converting those to some fake kconfig options
> would solve it, but it still seems like kconfig is stored in the
> global map and kconfig entries are resolved dynamically.
>
> Proposal:
>
> Resolve kconfig options statically upon loading. Basically rewrite
> ld+ldx to two nops and 'mov val, x'.
>
> I'm also trying to rewrite conditional jump when the condition is
> !imm. This seems to be catching all the cases in my program, but
> it's probably too hacky.
>
> I've attached very raw RFC patch to demonstrate the idea. Anything
> I'm missing? Any potential problems with this approach?
Have you considered using global variables for that?
With skeleton the user space has a natural way to set
all of these knobs after doing skel_open and before skel_load.
Then the verifier sees them as readonly vars and
automatically converts LDX into fixed constants and if the code
looks like if (my_config_var) then the verifier will remove
all the dead code too.
