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

From: Leo Yan <leo.yan@linaro.org>
To: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>,
	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>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Mathieu Poirier <mathieu.poirier@linaro.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Mike Leach <mike.leach@linaro.org>,
	linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
	coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v4 10/11] perf env: Set flag for kernel is 64-bit mode
Date: Tue, 13 Jul 2021 23:09:53 +0800	[thread overview]
Message-ID: <20210713150953.GC748506@leoy-ThinkPad-X240s> (raw)
In-Reply-To: <YOyGi9Zdrio7OmBX@kernel.org>

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@linaro.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