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

[Daniel Borkmann]

On 11/15/21 5:44 AM, Kajol Jain wrote:
> Branch data available to bpf programs can be very useful to get
> stack traces out of userspace applications.
>
> Commit fff7b64355ea ("bpf: Add bpf_read_branch_records() helper")
> added bpf support to capture branch records in x86. Enable this feature
> for powerpc as well.
>
> Commit 67306f84ca78 ("selftests/bpf: Add bpf_read_branch_records()
> selftest") adds selftest corresponding to bpf branch read
> function bpf_read_branch_records(). Used this selftest to
> test bpf support, for reading branch records in powerpc.
>
> Selftest result in power9 box before this patch changes:
>
> [command]# ./test_progs -t perf_branches
> Failed to load bpf_testmod.ko into the kernel: -8
> WARNING! Selftests relying on bpf_testmod.ko will be skipped.
> test_perf_branches_common:PASS:test_perf_branches_load 0 nsec
> test_perf_branches_common:PASS:attach_perf_event 0 nsec
> test_perf_branches_common:PASS:set_affinity 0 nsec
> check_good_sample:PASS:output not valid 0 nsec
> check_good_sample:FAIL:read_branches_size err -2
> check_good_sample:FAIL:read_branches_stack err -2
> check_good_sample:FAIL:read_branches_stack stack bytes written=-2
> not multiple of struct size=24
> check_good_sample:FAIL:read_branches_global err -2
> check_good_sample:FAIL:read_branches_global global bytes written=-2
> not multiple of struct size=24
> check_good_sample:PASS:read_branches_size 0 nsec
>   #75/1 perf_branches_hw:FAIL
>   #75/2 perf_branches_no_hw:OK
>   #75 perf_branches:FAIL
> Summary: 0/1 PASSED, 0 SKIPPED, 2 FAILED
>
> Selftest result in power9 box after this patch changes:
>
> [command]#: ./test_progs -t perf_branches
>   #75/1 perf_branches_hw:OK
>   #75/2 perf_branches_no_hw:OK
>   #75 perf_branches:OK
> Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED
>
> Signed-off-by: Kajol Jain<kjain@xxxxxxxxxxxxx>
> ---
>   kernel/trace/bpf_trace.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> 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?

>   	return -ENOENT;
>   #else
>   	static const u32 br_entry_size = sizeof(struct perf_branch_entry);