Re: [PATCH v2 2/4] perf tools: Use dynamic register set for Dwarf unwind

From: James Clark <james.clark@arm.com>
To: Leo Yan <leo.yan@linaro.org>
Cc: linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
	broonie@kernel.org, acme@kernel.org, german.gomez@arm.com,
	mathieu.poirier@linaro.org, john.garry@huawei.com,
	Will Deacon <will@kernel.org>, Mike Leach <mike.leach@linaro.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@kernel.org>, Namhyung Kim <namhyung@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 2/4] perf tools: Use dynamic register set for Dwarf unwind
Date: Wed, 18 May 2022 14:25:38 +0100	[thread overview]
Message-ID: <8f48b4d3-b463-a858-c0fc-0d503eee42dd@arm.com> (raw)
In-Reply-To: <20220517110322.GC153558@leoy-ThinkPad-X240s>

On 17/05/2022 12:03, Leo Yan wrote:
> On Tue, May 17, 2022 at 11:20:03AM +0100, James Clark wrote:
>> Architectures can detect availability of extra registers at
>> runtime so use this more complete set for unwinding. This
>> will include the VG register on arm64 in a later commit.
>>
>> If the function isn't implemented then PERF_REGS_MASK is
>> returned and there is no change.
>>
>> Signed-off-by: James Clark <james.clark@arm.com>
> 
> This patch looks good to me:
> Reviewed-by: Leo Yan <leo.yan@linaro.org>
> 
> Just curious, do you think should update the test (e.g.
> arch/arm64/tests/dwarf-unwind.c) to use arch__user_reg_mask()?

I don't think so because the normal set of registers is manually
loaded in tools/perf/arch/arm64/tests/regs_load.S so it wouldn't
include this pseudo register. Also there is no SVE in the call
chain of the test so it would never have an effect.

I could add a new test for SVE, but it depends on getting the
libunwind changes through first so will have to come later.

Thanks,
James

> 
> Thanks,
> Leo
> 
>> ---
>>  tools/perf/util/evsel.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
>> index 5fd7924f8eb3..787bbcbcd2ae 100644
>> --- a/tools/perf/util/evsel.c
>> +++ b/tools/perf/util/evsel.c
>> @@ -896,7 +896,7 @@ static void __evsel__config_callchain(struct evsel *evsel, struct record_opts *o
>>  					   "specifying a subset with --user-regs may render DWARF unwinding unreliable, "
>>  					   "so the minimal registers set (IP, SP) is explicitly forced.\n");
>>  			} else {
>> -				attr->sample_regs_user |= PERF_REGS_MASK;
>> +				attr->sample_regs_user |= arch__user_reg_mask();
>>  			}
>>  			attr->sample_stack_user = param->dump_size;
>>  			attr->exclude_callchain_user = 1;
>> -- 
>> 2.28.0
>>