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 22:41:58 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     James Clark <james.clark@....com>
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

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.

Thanks a lot for the reviewing and testing!
Leo