Re: [PATCH] perf/core: Reset remaining bits in perf_clear_branch_entry_bitfields()

[James Clark <james.clark@arm.com>]

On 01/12/2022 05:51, Anshuman Khandual wrote:
> perf_clear_branch_entry_bitfields() resets all struct perf_branch_entry bit
> fields before capturing branch records. This resets remaining bit fields
> except 'new_type', which is valid only when 'type' is PERF_BR_EXTEND_ABI.
> 
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: linux-perf-users@vger.kernel.org>
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
> This applies on v6.1-rc6
> 
> 'perf_branch_entry.new_type' can remain uninitialized as explained earlier.
> Also there is no PERF_BR_NEW_UNKNOWN to spare, because 'perf_branch_entry.
> new_type' enumeration starts at PERF_BR_NEW_FAULT_ALGN, to save a position
> for the extended branch types instead.
> 
>  include/linux/perf_event.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 0031f7b4d9ab..c97b5f6f77a4 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1110,8 +1110,9 @@ static inline void perf_clear_branch_entry_bitfields(struct perf_branch_entry *b
>  	br->in_tx = 0;
>  	br->abort = 0;
>  	br->cycles = 0;
> -	br->type = 0;
> +	br->type = PERF_BR_UNKNOWN;
>  	br->spec = PERF_BR_SPEC_NA;
> +	br->priv = PERF_BR_PRIV_UNKNOWN;
>  	br->reserved = 0;
>  }

I would vote for just memsetting the whole struct to 0 at this point and
making it work by ensuring the cleared from and to values are only set
after this function.

Or do the thing where it's wrapped in a union and the 'u64 value' member
is assigned 0. See union perf_mem_data_src. I don't know if this would
be a breaking change, but it doesn't look like it.

Currently this is a bit too fragile and the kind of bugs it will cause
are almost undetectable.

But as my proposal is an extra change on top of this:

Reviewed-by: James Clark <james.clark@arm.com>

James

>