Messages in this thread | | | Date | Thu, 1 Dec 2022 10:24:09 +0000 | Subject | Re: [PATCH] perf/core: Reset remaining bits in perf_clear_branch_entry_bitfields() | From | James Clark <> |
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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
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