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

From: Leo Yan <leo.yan@linaro.org>
To: James Clark <james.clark@arm.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>,
	Al Grant <Al.Grant@arm.com>, John Garry <john.garry@huawei.com>,
	Will Deacon <will@kernel.org>,
	Mathieu Poirier <mathieu.poirier@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@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Dave Martin <Dave.Martin@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 1/6] perf arm-spe: Remove unused enum value ARM_SPE_PER_CPU_MMAPS
Date: Thu, 15 Apr 2021 22:41:58 +0800	[thread overview]
Message-ID: <20210415144158.GD1011890@leoy-ThinkPad-X240s> (raw)
In-Reply-To: <882e13d1-c793-88c6-0b1e-629462fb99ac@arm.com>

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