linux-kernel - Re: [PATCH v2] perf: Support for Arm A32/T32 instruction sets in CoreSight trace

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Date:   Wed, 29 Aug 2018 15:34:16 +0100
From:   Robert Walker <robert.walker@....com>
To:     Kim Phillips <kim.phillips@....com>
Cc:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>, mike.leach@...aro.org,
        coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] perf: Support for Arm A32/T32 instruction sets in
 CoreSight trace

Hi Kim,


On 29/08/18 14:49, Kim Phillips wrote:
> On Wed, 29 Aug 2018 10:44:23 +0100
> Robert Walker <robert.walker@....com> wrote:
>
>> This patch adds support for generating instruction samples from trace of
>> AArch32 programs using the A32 and T32 instruction sets.
>>
>> T32 has variable 2 or 4 byte instruction size, so the conversion between
>> addresses and instruction counts requires extra information from the trace
>> decoder, requiring version 0.9.1 of OpenCSD.  A check for the new struct
>> member has been added to the feature check for OpenCSD.
>>
>> Signed-off-by: Robert Walker <robert.walker@....com>
>> ---
> ...
>> +++ b/tools/build/feature/test-libopencsd.c
>> @@ -3,6 +3,13 @@
>>   
>>   int main(void)
>>   {
>> +	/*
>> +	 * Requires ocsd_generic_trace_elem.num_instr_range introduced in
>> +	 * OpenCSD 0.9
> 0.9 != 0.9.1 in the above commit text: which is it?
I'll change it to 0.9.1 if there's another version of the patch (it was 
introduced in 0.9, but 0.9.1 has a necessary bug fix)
>> +	 */
>> +	ocsd_generic_trace_elem elem;
>> +	(void)elem.num_instr_range;
>> +
> This breaks building against older versions of OpenCSD, right?
>
>>   	(void)ocsd_get_version();
> Why don't we maintain building against older versions of the library,
> and use the version information to make the decision on whether to use
> the new feature being introduced in this patch?
The intention is to fail the feature detection check if the older 
version is installed - perf will still compile, but without the 
CoreSight trace support.

OpenCSD is still in development, so new features like this are being 
added and it would add a lot of #ifdef mess to the perf code to continue 
to support any machines with the old library version installed - there 
will only be a handful of machines affected and it's trivial to upgrade 
them (the new Debian packages are available).  How long would we 
continue to support such an older version?  I also don't see any 
precedent for supporting multiple dependent library versions in perf.
>
> Thanks,
>
> Kim
Regards

Rob