Re: [PATCH v1 11/11] perf build: Rename PERF_HAVE_DWARF_REGS to PERF_HAVE_LIBDW_REGS

[Ian Rogers]

On Sat, Sep 28, 2024 at 7:35 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> On Fri, 27 Sep 2024 11:15:21 -0700
> Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> > On Fri, Sep 27, 2024 at 10:16 AM Namhyung Kim <namhyung@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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.

The code is not making a statement about the DWARF standard, take arch/csky:
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/arch/csky/Makefile?h=perf-tools-next
```
# SPDX-License-Identifier: GPL-2.0-only
ifndef NO_DWARF
PERF_HAVE_DWARF_REGS := 1
endif
```
in the patch series NO_DWARF becomes NO_LIBDW, so it is now:
```
# SPDX-License-Identifier: GPL-2.0-only
ifndef NO_LIBDW
PERF_HAVE_DWARF_REGS := 1
endif
```
So the Makefile says that PERF_HAVE_DWARF_REGS is dependent on having
NO_LIBDW, that is having libdw implies PERF_HAVE_DWARF_REGS is defined
for csky.

Dwarf in the code base implies libdw, libunwind and potentially other
dwarf capable things like llvm. If we don't have libdw then NO_LIBDW
will be set and PERF_HAVE_DWARF_REGS won't be set. That is the more
general dwarf thing will not be set because of missing libdw. This
goes contrary to wanting this to be true whenever a dwarf thing is
present - something that reflecting what the standard says would
achieve.

In the code base right now PERF_HAVE_DWARF_REGS isn't a dwarf
dependent thing, it is a libdw dependent thing, this is why
PERF_HAVE_LIBDW_REGS is a more intention revealing name as it makes
the connection explicit.

We could change the code so that in Makefile.config we set something like:
```
...
ifndef NO_LIBDW
  PERF_HAVE_DWARF := 1
...
```
and in the arch/.../Makefiles change them to be:
```
if PERF_HAVE_DWARF
PERF_HAVE_DWARF_REGS := 1
endif
```
but this is going beyond the clean up this patch series was trying to
achieve. I also don't know of an architecture where dwarf is present
but registers are not, so having a definition for this case feels
redundant.

Thanks,
Ian

> 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@xxxxxxxxxx>