linux-kernel - Re: [PATCH v4 1/6] perf arm-spe: Remove unused enum value ARM_SPE_PER_CPU

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Date:   Thu, 15 Apr 2021 17:49:45 +0300
From:   James Clark <james.clark@....com>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Al Grant <Al.Grant@....com>,
        John Garry <john.garry@...wei.com>,
        Will Deacon <will@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/6] perf arm-spe: Remove unused enum value
 ARM_SPE_PER_CPU_MMAPS



On 15/04/2021 17:41, Leo Yan wrote:
> Hi James,
> 
> On Thu, Apr 15, 2021 at 05:13:36PM +0300, James Clark wrote:
>> On 12/04/2021 12:10, Leo Yan wrote:
>>> The enum value 'ARM_SPE_PER_CPU_MMAPS' is never used so remove it.
>>
>> Hi Leo,
>>
>> I think this causes an error when attempting to open a newly recorded file
>> with an old version of perf. The value ARM_SPE_AUXTRACE_PRIV_MAX is used here:
>>
>> 	size_t min_sz = sizeof(u64) * ARM_SPE_AUXTRACE_PRIV_MAX;
>> 	struct perf_record_time_conv *tc = &session->time_conv;
>> 	struct arm_spe *spe;
>> 	int err;
>>
>> 	if (auxtrace_info->header.size < sizeof(struct perf_record_auxtrace_info) +
>> 					min_sz)
>> 		return -EINVAL;
>>
>> And removing ARM_SPE_PER_CPU_MMAPS changes the value of ARM_SPE_AUXTRACE_PRIV_MAX.
>>
>> At least I think that's what's causing the problem. I get this error:
>>
>> 	./perf report -i per-thread-spe-time.data
>> 	0x1c0 [0x18]: failed to process type: 70 [Invalid argument]
>> 	Error:
>> 	failed to process sample
>> 	# To display the perf.data header info, please use --header/--header-only options.
>> 	#
> 
> Yes, when working on this patch I had concern as well.
> 
> I carefully thought that the perf tool should be backwards-compatible,
> but there have no requirement for forwards-compatibility.  This is the
> main reason why I kept this patch.
> 
> If you or anyone could confirm the forwards-compatibility is required,
> it's quite fine for me to drop this patch.
> 

Personally, I can easily imagine sending a file to someone to open with an older version and it causing
friction where it could be easily avoided. And it even made testing a bit more difficult because
I wanted to compare opening the same file with the patched and un-patched version. But if there
is no hard requirement I can't really put too much pressure to not remove it.

> Thanks a lot for the reviewing and testing!
> Leo
>