linux-kernel - Re: [PATCH v2] bpf: Remove config check to enable bpf support for branch records

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Date:   Thu, 18 Nov 2021 14:48:14 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Kajol Jain <kjain@...ux.ibm.com>
Cc:     bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Ziljstra <peterz@...radead.org>,
        Song Liu <songliubraving@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Yonghong Song <yhs@...com>,
        john fastabend <john.fastabend@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        KP Singh <kpsingh@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>, maddy@...ux.ibm.com,
        atrajeev@...ux.vnet.ibm.com,
        "linux-perf-use." <linux-perf-users@...r.kernel.org>,
        rnsastry@...ux.ibm.com
Subject: Re: [PATCH v2] bpf: Remove config check to enable bpf support for
 branch records

On Thu, Nov 18, 2021 at 5:10 AM Kajol Jain <kjain@...ux.ibm.com> wrote:
>
> Branch data available to bpf programs can be very useful to get
> stack traces out of userspace application.
>
> Commit fff7b64355ea ("bpf: Add bpf_read_branch_records() helper")
> added bpf support to capture branch records in x86. Enable this feature
> for other architectures as well by removing check specific to x86.
> Incase any platform didn't support branch stack, it will return with
> -EINVAL.
>
> Selftest 'perf_branches' result on power9 machine with branch stacks
> support.
>
> Before this patch changes:
> [command]# ./test_progs -t perf_branches
>  #88/1 perf_branches/perf_branches_hw:FAIL
>  #88/2 perf_branches/perf_branches_no_hw:OK
>  #88 perf_branches:FAIL
> Summary: 0/1 PASSED, 0 SKIPPED, 1 FAILED
>
> After this patch changes:
> [command]# ./test_progs -t perf_branches
>  #88/1 perf_branches/perf_branches_hw:OK
>  #88/2 perf_branches/perf_branches_no_hw:OK
>  #88 perf_branches:OK
> Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED
>
> Selftest 'perf_branches' result on power9 machine which doesn't
> support branch stack
>
> After this patch changes:
> [command]# ./test_progs -t perf_branches
>  #88/1 perf_branches/perf_branches_hw:SKIP
>  #88/2 perf_branches/perf_branches_no_hw:OK
>  #88 perf_branches:OK
> Summary: 1/1 PASSED, 1 SKIPPED, 0 FAILED
>
> Fixes: fff7b64355eac ("bpf: Add bpf_read_branch_records() helper")
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Kajol Jain <kjain@...ux.ibm.com>
> ---
>
> Tested this patch changes on power9 machine using selftest
> 'perf branches' which is added in commit 67306f84ca78 ("selftests/bpf:
> Add bpf_read_branch_records()")
>
> Changelog:
> v1 -> v2
> - Inorder to add bpf support to capture branch record in
>   powerpc, rather then adding config for powerpc, entirely
>   remove config check from bpf_read_branch_records function
>   as suggested by Peter Zijlstra

what will be returned for architectures that don't support branch
records? Will it be zero instead of -ENOENT?

>
> - Link to the v1 patch: https://lkml.org/lkml/2021/11/14/434
>
>  kernel/trace/bpf_trace.c | 4 ----
>  1 file changed, 4 deletions(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 7396488793ff..5e445985c6b4 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1402,9 +1402,6 @@ 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
> -       return -ENOENT;
> -#else
>         static const u32 br_entry_size = sizeof(struct perf_branch_entry);
>         struct perf_branch_stack *br_stack = ctx->data->br_stack;
>         u32 to_copy;
> @@ -1425,7 +1422,6 @@ BPF_CALL_4(bpf_read_branch_records, struct bpf_perf_event_data_kern *, ctx,
>         memcpy(buf, br_stack->entries, to_copy);
>
>         return to_copy;
> -#endif
>  }
>
>  static const struct bpf_func_proto bpf_read_branch_records_proto = {
> --
> 2.27.0
>