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

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Date:   Tue, 16 Nov 2021 20:51:35 +0530
From:   kajoljain <kjain@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org, acme@...nel.org,
        songliubraving@...com, andrii@...nel.org, kafai@...com, yhs@...com,
        john.fastabend@...il.com, davem@...emloft.net, kpsingh@...nel.org,
        hawk@...nel.org, kuba@...nel.org, maddy@...ux.ibm.com,
        atrajeev@...ux.vnet.ibm.com, linux-perf-users@...r.kernel.org,
        rnsastry@...ux.ibm.com
Subject: Re: [PATCH] bpf: Enable bpf support for reading branch records in
 powerpc



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
function.

Thanks,
Kajol Jain
> 
>