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

[Namhyung Kim]

On Thu, Oct 03, 2024 at 05:58:13PM -0700, Ian Rogers wrote:
> On Thu, Oct 3, 2024 at 3:48 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> > I agree renaming libdw-specific parts.  But the register is for DWARF,
> > not libdw even if it's currently used by libdw only.   So I don't want
> > to rename it.
> 
> So your objection is that we have files called:
> tools/perf/arch/*/util/dwarf-regs.c
> and PERF_HAVE_DRWARF_REGS is an indication that this file exists. This
> file declares a single get_arch_regnum function. The building of the
> file after this series is:
> perf-util-$(CONFIG_LIBDW)     += dwarf-regs.o

Well.. I think we can even make it

perf-util-y += dwarf-regs.o

since it doesn't have any dependency on libdw.  But it'd be inefficent
to ship the dead code and data.  Anyway we may remove the condition to
define the PERF_HAVE_DWARF_REGS like below.

diff --git a/tools/perf/arch/x86/Makefile b/tools/perf/arch/x86/Makefile
index 67b4969a673836eb..f1eb1ee1ea25ca53 100644
--- a/tools/perf/arch/x86/Makefile
+++ b/tools/perf/arch/x86/Makefile
@@ -1,7 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0
-ifndef NO_DWARF
 PERF_HAVE_DWARF_REGS := 1
-endif
 HAVE_KVM_STAT_SUPPORT := 1
 PERF_HAVE_ARCH_REGS_QUERY_REGISTER_OFFSET := 1
 PERF_HAVE_JITDUMP := 1

> 
> My objection is that PERF_HAVE_DWARF_REGS is controlling the #define
> HAVE_LIBDW_SUPPORT, so dwarf (that can mean libunwind, libdw, etc.) is
> guarding having libdw which is backward and part of what this series
> has been trying to clean up.

Why not?  If the arch doesn't define DWARF registers, it can refuse
libdw support because it won't work well.

> 
> If we rename tools/perf/arch/*/util/dwarf-regs.c to
> tools/perf/arch/*/util/libdw-helpers.c the PERF_HAVE_DWARF_REGS can be
> renamed to PERF_HAVE_LIBDW_HELPERS to align. Then
> PERF_HAVE_LIBDW_HELPERS guarding the #define PERF_HAVE_LIBDW makes
> sense to me and I think we achieve the filename alignment you are
> looking for.

I don't think it's a good idea.  The logic is not specific to libdw.

> 
> Yes get_arch_regnum could make sense out of libdw and needn't just be
> a helper for it, but let's worry about that when there's a need.
> What's confusing at the moment is does libdw provide dwarf support,
> which I'd say is expected, or does dwarf provide libdw support?

As I said, it's about refusing libdw.

  ifndef NO_LIBDW
    ifeq ($(origin PERF_HAVE_DWARF_REGS), undefined)
      $(warning DWARF register mappings have not been defined for architecture $(SRCARCH), DWARF support disabled)
      NO_LIBDW := 1
    else
      CFLAGS += -DHAVE_DWARF_SUPPORT $(LIBDW_CFLAGS)
      LDFLAGS += $(LIBDW_LDFLAGS)
      EXTLIBS += ${DWARFLIBS}
      $(call detected,CONFIG_DWARF)
    endif # PERF_HAVE_DWARF_REGS
  endif # NO_LIBDW

Thanks,
Namhyung