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

["Hunter, Adrian" <adrian.hunter@intel.com>]

> -----Original Message-----
> From: Leo Yan <leo.yan@linaro.org>
> Sent: Tuesday, July 13, 2021 6:10 PM
> To: Arnaldo Carvalho de Melo <acme@kernel.org>
> Cc: Hunter, Adrian <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
> 
> 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.

I think we store the arch, so if the "kernel_is_64_bit" calculation depends only on arch
then I guess we don't need a new feature at the moment.

> 
> 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