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

From: kajoljain
Date: Tue Nov 16 2021 - 10:22:42 EST

On 11/16/21 2:04 PM, Peter Zijlstra wrote:
> 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?

Hi Peterz/Daniel,
Thanks for reviewing the patch

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

I agree, it make more sense to entirely remove this arch check from
here. Because anyway, incase any arch doesn't support this
functionality, bpf_read_branch_records will return -EINVAL.

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

In this patch, we are basically adding powerpc support to capture
branch records via bpf_read_branch_records function. We are still
looking into adding support for perf_snapshot_branch_stack for powerpc.

I will send a follow up to remove arch check in bpf_read_branch_records

Kajol Jain