Re: [RFC bpf-next v1 3/8] bpf, x86: no_caller_saved_registers for bpf_get_smp_processor_id()

Eduard Zingerman
2024-07-02T20:44:01Z

On Mon, 2024-07-01 at 17:41 -0700, Andrii Nakryiko wrote:

[...]

> > @@ -158,6 +158,7 @@ const struct bpf_func_proto bpf_get_smp_processor_id_proto = {
> > .func = bpf_get_smp_processor_id,
> > .gpl_only = false,
> > .ret_type = RET_INTEGER,
> > + .nocsr = true,
>
> I'm wondering if we should call this flag in such a way that it's
> clear that this is more of an request, while the actual nocsr cleanup
> and stuff is done only if BPF verifier/BPF JIT support that for
> specific architecture/config/etc?

Can change to .allow_nocsr. On the other hand, can remove this flag
completely and rely on call_csr_mask().

[...]

> > @@ -16030,7 +16030,14 @@ static u8 get_helper_reg_mask(const struct bpf_func_proto *fn)
> > */
> > static bool verifier_inlines_helper_call(struct bpf_verifier_env *env, s32 imm)
> > {
> > - return false;
> > + switch (imm) {
> > +#ifdef CONFIG_X86_64
> > + case BPF_FUNC_get_smp_processor_id:
> > + return env->prog->jit_requested && bpf_jit_supports_percpu_insn();
> > +#endif
>
> please see bpf_jit_inlines_helper_call(), arm64 and risc-v inline it
> in JIT, so we need to validate they don't assume any of R1-R5 register
> to be a scratch register

At the moment I return false for this archs.
Or do you suggest these to be added in the current patch-set? …