linux-kernel - Re: [PATCH v1 11/11] perf build: Rename PERF_HAVE_DWARF_REGS to PERF_HAVE_LIBDW

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Message-Id: <20240929113521.9b7e8fd67af154520e2c9d8e@kernel.org>
Date: Sun, 29 Sep 2024 11:35:21 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Namhyung Kim <namhyung@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de
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Subject: Re: [PATCH v1 11/11] perf build: Rename PERF_HAVE_DWARF_REGS to
 PERF_HAVE_LIBDW_REGS

On Fri, 27 Sep 2024 11:15:21 -0700
Ian Rogers <irogers@...gle.com> wrote:

> On Fri, Sep 27, 2024 at 10:16 AM Namhyung Kim <namhyung@...nel.org> wrote:
> >
> > On Thu, Sep 26, 2024 at 12:55:18PM -0700, Ian Rogers wrote:
> > > On Thu, Sep 26, 2024 at 12:40 PM Namhyung Kim <namhyung@...nel.org> wrote:
> > > >
> > > > On Thu, Sep 26, 2024 at 05:47:16AM -0700, Ian Rogers wrote:
> > > > > On Wed, Sep 25, 2024 at 8:27 PM Namhyung Kim <namhyung@...nel.org> wrote:
> > > > > >
> > > > > > On Tue, Sep 24, 2024 at 09:04:18AM -0700, Ian Rogers wrote:
> > > > > > > The name dwarf can imply libunwind support, whereas
> > > > > > > PERF_HAVE_DWARF_REGS is only enabled with libdw support. Rename to
> > > > > > > make it clearer there is a libdw connection.
> > > > > >
> > > > > > While it only covers libdw, I think the idea of this macro is whether
> > > > > > the arch has register mappings defined in DWARF standard.  So I think
> > > > > > it's better to keep the name for this case.
> > > > >
> > > > > How can the dwarf standard exist for an arch but not define registers?
> > > >
> > > > I meant it's about the arch code in the perf tools to have the mapping,
> > > > not the DWARF standard itself.
> > >
> > > But we guard those definitions behind having libdw:
> > > https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/arch/x86/Makefile?h=perf-tools-next#n3
> > > So we only have the regs if libdw is present, not if dwarf is in use
> > > for libunwind/libdw. Hence wanting to be specific that they are just a
> > > libdw and not a dwarf thing. Trying to use the regs in libunwind code
> > > would be broken. That could change but I wanted to make the code clear
> > > for the way things are at the moment.
> >
> > I understand your point but calling it LIBDW_REGS looks unnatural to me.
> 
> I don't follow. Wouldn't it be unnatural to see PERF_HAVE_DWARF_REGS
> in libunwind code but you are to some how know that the code only had
> meaning if libdw was present? I don't like the implication that DWARF
> means LIBDW as throughout the code it doesn't. I think the name
> PERF_HAVE_LIBDW_REGS better captures how the code is, makes the code
> more intention revealing and so readable, etc.

I agree with Namhyung this point. dwarf-regs is defined only by the
DWARF standard, not libdw only. The standard encode registers by a digit
number and the dwarf-regs decode the number to actual register name.

Actually, there is a histrical reason I had called it is DWARF. I used to
use "libdwarf", and switched to "libdw" for required features. So I know
there are more than 1 implementation of DWARF library, and the libdwarf
also uses the same operation number because it depends on the same standard.

https://github.com/davea42/libdwarf-code/blob/main/src/lib/libdwarf/dwarf.h#L809

So I think we'd better keep it call as DWARF_REGS.

Thank you,

> 
> Thanks,
> Ian
> 

-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>