Re: [PATCH] bpf: Enable bpf support for reading branch records in powerpc

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Nov 16, 2021 at 12:30:07AM +0100, Daniel Borkmann wrote:

> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index fdd14072fc3b..2b7343b64bb7 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -1245,7 +1245,7 @@ static const struct bpf_func_proto bpf_perf_prog_read_value_proto = {
> >   BPF_CALL_4(bpf_read_branch_records, struct bpf_perf_event_data_kern *, ctx,
> >   	   void *, buf, u32, size, u64, flags)
> >   {
> > -#ifndef CONFIG_X86
> > +#if !(defined(CONFIG_X86) || defined(CONFIG_PPC64))
> 
> Can this really be enabled generically? Looking at 3925f46bb590 ("powerpc/perf: Enable
> branch stack sampling framework") it says POWER8 [and beyond]. Should there be a generic
> Kconfig symbol like ARCH_HAS_BRANCH_RECORDS that can be selected by archs instead?

I conplained about it before as well. I'd just take it out entirely.

If perf_snapshot_branch_stack isn't implemnted it'll return 0 and then
we'll -Esomething anyway.