linux-kernel - Re: [PATCH v4 10/11] perf env: Set flag for kernel is 64-bit mode

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Date:   Tue, 13 Jul 2021 23:09:53 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Adrian Hunter <adrian.hunter@...el.com>,
        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>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mike Leach <mike.leach@...aro.org>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 10/11] perf env: Set flag for kernel is 64-bit mode

Hi Arnaldo, Adrian,

On Mon, Jul 12, 2021 at 03:14:35PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Sun, Jul 11, 2021 at 06:41:04PM +0800, Leo Yan escreveu:
> > It's useful to know that the kernel is running in 32-bit or 64-bit
> > mode.  E.g. We can decide if perf tool is running in compat mode
> > from this info.
> > 
> > This patch adds a global variable "kernel_is_64_bit", it's initialized
> > when a session setups environment, its value is decided by checking the
> > architecture string.
> > 
> > Signed-off-by: Leo Yan <leo.yan@...aro.org>
> > ---
> >  tools/perf/util/env.c | 17 ++++++++++++++++-
> >  tools/perf/util/env.h |  1 +
> >  2 files changed, 17 insertions(+), 1 deletion(-)
> > 
> > diff --git a/tools/perf/util/env.c b/tools/perf/util/env.c
> > index ebc5e9ad35db..345635a2e842 100644
> > --- a/tools/perf/util/env.c
> > +++ b/tools/perf/util/env.c
> > @@ -11,6 +11,7 @@
> >  #include <stdlib.h>
> >  #include <string.h>
> >  
> > +int kernel_is_64_bit;
> >  struct perf_env perf_env;
> 
> Why can't this be in 'struct perf_env'?

Good question.  I considered to add it in struct perf_env but finally
I used this way; the reason is this variable "kernel_is_64_bit" is only
used during recording phase for AUX ring buffer, and don't use it for
report.  So seems to me it's over complexity to add a new field and
just wander if it's necessary to save this field as new feature in the
perf header.

Combining the comment from Adrian in another email, I think it's good
to add a new field "compat_mode" in the struct perf_env, and this field
will be initialized in build-record.c.  Currently we don't need to save
this value into the perf file, if later we need to use this value for
decoding phase, then we can add a new feature item to save "compat_mode"
into the perf file's header.

If you have any different idea, please let me know.  Thanks!

Leo