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

Andrii Nakryiko
2024-07-02T21:12:00Z

On Tue, Jul 2, 2024 at 1:44 PM Eduard Zingerman <eddyz87@gmail.com> wrote:
>
> 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().

I like the declaration that helper is eligible to be close to helper
definition, so I'd definitely keep it, but yeah "allow_nocsr" seems
betterto me

>
> [...]
>
> > > @@ -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?

I'd add them from the get go. CC Puranjay to double-check?

>
> …