| Message ID | 20210130004401.2528717-2-ndesaulniers@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Kbuild: DWARF v5 support | expand |
On Sat, Jan 30, 2021 at 10:52 AM Nathan Chancellor <nathan@kernel.org> wrote: > > On Fri, Jan 29, 2021 at 04:44:00PM -0800, Nick Desaulniers wrote: > > Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice which is > > the default. Does so in a way that's forward compatible with existing > > configs, and makes adding future versions more straightforward. > > > > GCC since ~4.8 has defaulted to this DWARF version implicitly. > > > > Suggested-by: Arvind Sankar <nivedita@alum.mit.edu> > > Suggested-by: Fangrui Song <maskray@google.com> > > Suggested-by: Nathan Chancellor <nathan@kernel.org> > > Suggested-by: Masahiro Yamada <masahiroy@kernel.org> > > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> > > One comment below: > > Reviewed-by: Nathan Chancellor <nathan@kernel.org> > > > --- > > Makefile | 5 ++--- > > lib/Kconfig.debug | 16 +++++++++++----- > > 2 files changed, 13 insertions(+), 8 deletions(-) > > > > diff --git a/Makefile b/Makefile > > index 95ab9856f357..d2b4980807e0 100644 > > --- a/Makefile > > +++ b/Makefile > > @@ -830,9 +830,8 @@ ifneq ($(LLVM_IAS),1) > > KBUILD_AFLAGS += -Wa,-gdwarf-2 > > It is probably worth a comment somewhere that assembly files will still > have DWARF v2. I agree. Please noting the reason will be helpful. Could you summarize Jakub's comment in short? https://patchwork.kernel.org/project/linux-kbuild/patch/20201022012106.1875129-1-ndesaulniers@google.com/#23727667 One more question. Can we remove -g option like follows? ifdef CONFIG_DEBUG_INFO_SPLIT DEBUG_CFLAGS += -gsplit-dwarf -else -DEBUG_CFLAGS += -g endif In the current mainline code, -g is the only debug option if CONFIG_DEBUG_INFO_DWARF4 is disabled. The GCC manual says: https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Debugging-Options.html#Debugging-Options -g Produce debugging information in the operating system’s native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information. Of course, we expect the -g option will produce the debug info in the DWARF format. With this patch set applied, it is very explicit. Only the format type, but also the version. The compiler will be given either -gdwarf-4 or -gdwarf-5, making the -g option redundant, I think. > > > endif > > > > -ifdef CONFIG_DEBUG_INFO_DWARF4 > > -DEBUG_CFLAGS += -gdwarf-4 > > -endif > > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4 > > +DEBUG_CFLAGS += -gdwarf-$(dwarf-version-y) > > > > ifdef CONFIG_DEBUG_INFO_REDUCED > > DEBUG_CFLAGS += $(call cc-option, -femit-struct-debug-baseonly) \ > > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug > > index e906ea906cb7..94c1a7ed6306 100644 > > --- a/lib/Kconfig.debug > > +++ b/lib/Kconfig.debug > > @@ -256,13 +256,19 @@ config DEBUG_INFO_SPLIT > > to know about the .dwo files and include them. > > Incompatible with older versions of ccache. > > > > +choice > > + prompt "DWARF version" > > + help > > + Which version of DWARF debug info to emit. > > + > > config DEBUG_INFO_DWARF4 > > - bool "Generate dwarf4 debuginfo" > > + bool "Generate DWARF Version 4 debuginfo" > > help > > - Generate dwarf4 debug info. This requires recent versions > > - of gcc and gdb. It makes the debug information larger. > > - But it significantly improves the success of resolving > > - variables in gdb on optimized code. > > + Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+. > > + It makes the debug information larger, but it significantly > > + improves the success of resolving variables in gdb on optimized code. > > + > > +endchoice # "DWARF version" > > > > config DEBUG_INFO_BTF > > bool "Generate BTF typeinfo" > > -- > > 2.30.0.365.g02bc693789-goog > >
On Thu, Feb 4, 2021 at 7:23 AM Masahiro Yamada <masahiroy@kernel.org> wrote: > > On Sat, Jan 30, 2021 at 10:52 AM Nathan Chancellor <nathan@kernel.org> wrote: > > > > On Fri, Jan 29, 2021 at 04:44:00PM -0800, Nick Desaulniers wrote: > > > Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice which is > > > the default. Does so in a way that's forward compatible with existing > > > configs, and makes adding future versions more straightforward. > > > > > > GCC since ~4.8 has defaulted to this DWARF version implicitly. > > > > > > Suggested-by: Arvind Sankar <nivedita@alum.mit.edu> > > > Suggested-by: Fangrui Song <maskray@google.com> > > > Suggested-by: Nathan Chancellor <nathan@kernel.org> > > > Suggested-by: Masahiro Yamada <masahiroy@kernel.org> > > > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> > > > > One comment below: > > > > Reviewed-by: Nathan Chancellor <nathan@kernel.org> > > > > > --- > > > Makefile | 5 ++--- > > > lib/Kconfig.debug | 16 +++++++++++----- > > > 2 files changed, 13 insertions(+), 8 deletions(-) > > > > > > diff --git a/Makefile b/Makefile > > > index 95ab9856f357..d2b4980807e0 100644 > > > --- a/Makefile > > > +++ b/Makefile > > > @@ -830,9 +830,8 @@ ifneq ($(LLVM_IAS),1) > > > KBUILD_AFLAGS += -Wa,-gdwarf-2 > > > > It is probably worth a comment somewhere that assembly files will still > > have DWARF v2. > > I agree. > Please noting the reason will be helpful. > > Could you summarize Jakub's comment in short? > https://patchwork.kernel.org/project/linux-kbuild/patch/20201022012106.1875129-1-ndesaulniers@google.com/#23727667 > > > > > > > One more question. > > > Can we remove -g option like follows? > > > ifdef CONFIG_DEBUG_INFO_SPLIT > DEBUG_CFLAGS += -gsplit-dwarf > -else > -DEBUG_CFLAGS += -g > endif > > > > > > In the current mainline code, > -g is the only debug option > if CONFIG_DEBUG_INFO_DWARF4 is disabled. > > > The GCC manual says: > https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Debugging-Options.html#Debugging-Options > > > -g > > Produce debugging information in the operating system’s > native format (stabs, COFF, XCOFF, or DWARF). > GDB can work with this debugging information. > > > Of course, we expect the -g option will produce > the debug info in the DWARF format. > > > > > > With this patch set applied, it is very explicit. > > Only the format type, but also the version. I mean: Not only the format type, but also the version is explicit. > The compiler will be given either > -gdwarf-4 or -gdwarf-5, > making the -g option redundant, I think. > > > > > >
On Wed, Feb 3, 2021 at 2:24 PM Masahiro Yamada <masahiroy@kernel.org> wrote: > > On Sat, Jan 30, 2021 at 10:52 AM Nathan Chancellor <nathan@kernel.org> wrote: > > > > On Fri, Jan 29, 2021 at 04:44:00PM -0800, Nick Desaulniers wrote: > > > Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice which is > > > the default. Does so in a way that's forward compatible with existing > > > configs, and makes adding future versions more straightforward. > > > > > > GCC since ~4.8 has defaulted to this DWARF version implicitly. > > > > > > Suggested-by: Arvind Sankar <nivedita@alum.mit.edu> > > > Suggested-by: Fangrui Song <maskray@google.com> > > > Suggested-by: Nathan Chancellor <nathan@kernel.org> > > > Suggested-by: Masahiro Yamada <masahiroy@kernel.org> > > > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> > > > > One comment below: > > > > Reviewed-by: Nathan Chancellor <nathan@kernel.org> > > > > > --- > > > Makefile | 5 ++--- > > > lib/Kconfig.debug | 16 +++++++++++----- > > > 2 files changed, 13 insertions(+), 8 deletions(-) > > > > > > diff --git a/Makefile b/Makefile > > > index 95ab9856f357..d2b4980807e0 100644 > > > --- a/Makefile > > > +++ b/Makefile > > > @@ -830,9 +830,8 @@ ifneq ($(LLVM_IAS),1) > > > KBUILD_AFLAGS += -Wa,-gdwarf-2 > > > > It is probably worth a comment somewhere that assembly files will still > > have DWARF v2. > > I agree. > Please noting the reason will be helpful. Via a comment in the source, or in the commit message? > > Could you summarize Jakub's comment in short? > https://patchwork.kernel.org/project/linux-kbuild/patch/20201022012106.1875129-1-ndesaulniers@google.com/#23727667 Via a comment in the source, or in the commit message? > One more question. > > > Can we remove -g option like follows? > > > ifdef CONFIG_DEBUG_INFO_SPLIT > DEBUG_CFLAGS += -gsplit-dwarf > -else > -DEBUG_CFLAGS += -g > endif > > > > > > In the current mainline code, > -g is the only debug option > if CONFIG_DEBUG_INFO_DWARF4 is disabled. > > > The GCC manual says: > https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Debugging-Options.html#Debugging-Options > > > -g > > Produce debugging information in the operating system’s > native format (stabs, COFF, XCOFF, or DWARF). > GDB can work with this debugging information. > > > Of course, we expect the -g option will produce > the debug info in the DWARF format. > > > > > > With this patch set applied, it is very explicit. > > Only the format type, but also the version. > > The compiler will be given either > -gdwarf-4 or -gdwarf-5, > making the -g option redundant, I think. Can I provide that as a separate patch? I don't want any breakage from that to delay DWARF v5 support further. If so, should it be the first patch or last?
On Thu, Feb 4, 2021 at 8:16 AM Nick Desaulniers <ndesaulniers@google.com> wrote: > > On Wed, Feb 3, 2021 at 2:24 PM Masahiro Yamada <masahiroy@kernel.org> wrote: > > > > On Sat, Jan 30, 2021 at 10:52 AM Nathan Chancellor <nathan@kernel.org> wrote: > > > > > > On Fri, Jan 29, 2021 at 04:44:00PM -0800, Nick Desaulniers wrote: > > > > Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice which is > > > > the default. Does so in a way that's forward compatible with existing > > > > configs, and makes adding future versions more straightforward. > > > > > > > > GCC since ~4.8 has defaulted to this DWARF version implicitly. > > > > > > > > Suggested-by: Arvind Sankar <nivedita@alum.mit.edu> > > > > Suggested-by: Fangrui Song <maskray@google.com> > > > > Suggested-by: Nathan Chancellor <nathan@kernel.org> > > > > Suggested-by: Masahiro Yamada <masahiroy@kernel.org> > > > > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> > > > > > > One comment below: > > > > > > Reviewed-by: Nathan Chancellor <nathan@kernel.org> > > > > > > > --- > > > > Makefile | 5 ++--- > > > > lib/Kconfig.debug | 16 +++++++++++----- > > > > 2 files changed, 13 insertions(+), 8 deletions(-) > > > > > > > > diff --git a/Makefile b/Makefile > > > > index 95ab9856f357..d2b4980807e0 100644 > > > > --- a/Makefile > > > > +++ b/Makefile > > > > @@ -830,9 +830,8 @@ ifneq ($(LLVM_IAS),1) > > > > KBUILD_AFLAGS += -Wa,-gdwarf-2 > > > > > > It is probably worth a comment somewhere that assembly files will still > > > have DWARF v2. > > > > I agree. > > Please noting the reason will be helpful. > > Via a comment in the source, or in the commit message? > > > > > Could you summarize Jakub's comment in short? > > https://patchwork.kernel.org/project/linux-kbuild/patch/20201022012106.1875129-1-ndesaulniers@google.com/#23727667 > > Via a comment in the source, or in the commit message? Both in the source if you can summarize it in three lines or so. If you need to add more detailed explanation, please provide it in the commit log. > > > > With this patch set applied, it is very explicit. > > > > Only the format type, but also the version. > > > > The compiler will be given either > > -gdwarf-4 or -gdwarf-5, > > making the -g option redundant, I think. > > Can I provide that as a separate patch? I don't want any breakage > from that to delay DWARF v5 support further. If so, should it be the > first patch or last? OK. At the last to be safe? I am fine with doing a clean-up work later. > -- > Thanks, > ~Nick Desaulniers -- Best Regards Masahiro Yamada
On 2021-02-04, Masahiro Yamada wrote: >On Sat, Jan 30, 2021 at 10:52 AM Nathan Chancellor <nathan@kernel.org> wrote: >> >> On Fri, Jan 29, 2021 at 04:44:00PM -0800, Nick Desaulniers wrote: >> > Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice which is >> > the default. Does so in a way that's forward compatible with existing >> > configs, and makes adding future versions more straightforward. >> > >> > GCC since ~4.8 has defaulted to this DWARF version implicitly. >> > >> > Suggested-by: Arvind Sankar <nivedita@alum.mit.edu> >> > Suggested-by: Fangrui Song <maskray@google.com> >> > Suggested-by: Nathan Chancellor <nathan@kernel.org> >> > Suggested-by: Masahiro Yamada <masahiroy@kernel.org> >> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> >> >> One comment below: >> >> Reviewed-by: Nathan Chancellor <nathan@kernel.org> >> >> > --- >> > Makefile | 5 ++--- >> > lib/Kconfig.debug | 16 +++++++++++----- >> > 2 files changed, 13 insertions(+), 8 deletions(-) >> > >> > diff --git a/Makefile b/Makefile >> > index 95ab9856f357..d2b4980807e0 100644 >> > --- a/Makefile >> > +++ b/Makefile >> > @@ -830,9 +830,8 @@ ifneq ($(LLVM_IAS),1) >> > KBUILD_AFLAGS += -Wa,-gdwarf-2 >> >> It is probably worth a comment somewhere that assembly files will still >> have DWARF v2. > >I agree. >Please noting the reason will be helpful. > >Could you summarize Jakub's comment in short? >https://patchwork.kernel.org/project/linux-kbuild/patch/20201022012106.1875129-1-ndesaulniers@google.com/#23727667 > > > > > > >One more question. > > >Can we remove -g option like follows? > > > ifdef CONFIG_DEBUG_INFO_SPLIT > DEBUG_CFLAGS += -gsplit-dwarf >-else >-DEBUG_CFLAGS += -g > endif GCC 11/Clang 12 -gsplit-dwarf no longer imply -g2 (https://reviews.llvm.org/D80391). May be worth checking whether -gsplit-dwarf is used without a debug info enabling option. > > > > >In the current mainline code, >-g is the only debug option >if CONFIG_DEBUG_INFO_DWARF4 is disabled. > > >The GCC manual says: >https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Debugging-Options.html#Debugging-Options > > >-g > > Produce debugging information in the operating system’s > native format (stabs, COFF, XCOFF, or DWARF). > GDB can work with this debugging information. > > >Of course, we expect the -g option will produce >the debug info in the DWARF format. > > > > > >With this patch set applied, it is very explicit. > >Only the format type, but also the version. > >The compiler will be given either >-gdwarf-4 or -gdwarf-5, >making the -g option redundant, I think. -gdwarf-N does imply -g2 but personally I'd not suggest remove it if it already exists. The non-orthogonality is the reason Clang has -fdebug-default-version (https://reviews.llvm.org/D69822). > > > > > > > > >> >> > endif >> > >> > -ifdef CONFIG_DEBUG_INFO_DWARF4 >> > -DEBUG_CFLAGS += -gdwarf-4 >> > -endif >> > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4 >> > +DEBUG_CFLAGS += -gdwarf-$(dwarf-version-y) >> > >> > ifdef CONFIG_DEBUG_INFO_REDUCED >> > DEBUG_CFLAGS += $(call cc-option, -femit-struct-debug-baseonly) \ >> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug >> > index e906ea906cb7..94c1a7ed6306 100644 >> > --- a/lib/Kconfig.debug >> > +++ b/lib/Kconfig.debug >> > @@ -256,13 +256,19 @@ config DEBUG_INFO_SPLIT >> > to know about the .dwo files and include them. >> > Incompatible with older versions of ccache. >> > >> > +choice >> > + prompt "DWARF version" >> > + help >> > + Which version of DWARF debug info to emit. >> > + >> > config DEBUG_INFO_DWARF4 >> > - bool "Generate dwarf4 debuginfo" >> > + bool "Generate DWARF Version 4 debuginfo" >> > help >> > - Generate dwarf4 debug info. This requires recent versions >> > - of gcc and gdb. It makes the debug information larger. >> > - But it significantly improves the success of resolving >> > - variables in gdb on optimized code. >> > + Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+. >> > + It makes the debug information larger, but it significantly >> > + improves the success of resolving variables in gdb on optimized code. >> > + >> > +endchoice # "DWARF version" >> > >> > config DEBUG_INFO_BTF >> > bool "Generate BTF typeinfo" >> > -- >> > 2.30.0.365.g02bc693789-goog >> > > > > >-- >Best Regards >Masahiro Yamada
On Wed, Feb 3, 2021 at 7:32 PM Fangrui Song <maskray@google.com> wrote: > > On 2021-02-04, Masahiro Yamada wrote: > >On Sat, Jan 30, 2021 at 10:52 AM Nathan Chancellor <nathan@kernel.org> wrote: > >> > >> On Fri, Jan 29, 2021 at 04:44:00PM -0800, Nick Desaulniers wrote: > >> > Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice which is > >> > the default. Does so in a way that's forward compatible with existing > >> > configs, and makes adding future versions more straightforward. > >> > > >> > GCC since ~4.8 has defaulted to this DWARF version implicitly. > >> > > >> > Suggested-by: Arvind Sankar <nivedita@alum.mit.edu> > >> > Suggested-by: Fangrui Song <maskray@google.com> > >> > Suggested-by: Nathan Chancellor <nathan@kernel.org> > >> > Suggested-by: Masahiro Yamada <masahiroy@kernel.org> > >> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> > >> > >> One comment below: > >> > >> Reviewed-by: Nathan Chancellor <nathan@kernel.org> > >> > >> > --- > >> > Makefile | 5 ++--- > >> > lib/Kconfig.debug | 16 +++++++++++----- > >> > 2 files changed, 13 insertions(+), 8 deletions(-) > >> > > >> > diff --git a/Makefile b/Makefile > >> > index 95ab9856f357..d2b4980807e0 100644 > >> > --- a/Makefile > >> > +++ b/Makefile > >> > @@ -830,9 +830,8 @@ ifneq ($(LLVM_IAS),1) > >> > KBUILD_AFLAGS += -Wa,-gdwarf-2 > >> > >> It is probably worth a comment somewhere that assembly files will still > >> have DWARF v2. > > > >I agree. > >Please noting the reason will be helpful. > > > >Could you summarize Jakub's comment in short? > >https://patchwork.kernel.org/project/linux-kbuild/patch/20201022012106.1875129-1-ndesaulniers@google.com/#23727667 > > > > > > > > > > > > > >One more question. > > > > > >Can we remove -g option like follows? > > > > > > ifdef CONFIG_DEBUG_INFO_SPLIT > > DEBUG_CFLAGS += -gsplit-dwarf > >-else > >-DEBUG_CFLAGS += -g > > endif > > GCC 11/Clang 12 -gsplit-dwarf no longer imply -g2 > (https://reviews.llvm.org/D80391). May be worth checking whether > -gsplit-dwarf is used without a debug info enabling option. Indeed, I also remember -g was required for -fno-eliminate-unused-debug-types, used by libabigail. https://reviews.llvm.org/D80242 Masahiro, respectfully, I will not touch removing -g in this series. I suspect it will be its own can of worms. > > > > > > > > > > >In the current mainline code, > >-g is the only debug option > >if CONFIG_DEBUG_INFO_DWARF4 is disabled. > > > > > >The GCC manual says: > >https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Debugging-Options.html#Debugging-Options > > > > > >-g > > > > Produce debugging information in the operating system’s > > native format (stabs, COFF, XCOFF, or DWARF). > > GDB can work with this debugging information. > > > > > >Of course, we expect the -g option will produce > >the debug info in the DWARF format. > > > > > > > > > > > >With this patch set applied, it is very explicit. > > > >Only the format type, but also the version. > > > >The compiler will be given either > >-gdwarf-4 or -gdwarf-5, > >making the -g option redundant, I think. > > -gdwarf-N does imply -g2 but personally I'd not suggest remove it if it > already exists. The non-orthogonality is the reason Clang has > -fdebug-default-version (https://reviews.llvm.org/D69822).
On Fri, Jan 29, 2021 at 5:52 PM Nathan Chancellor <nathan@kernel.org> wrote: > > On Fri, Jan 29, 2021 at 04:44:00PM -0800, Nick Desaulniers wrote: > > Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice which is > > the default. Does so in a way that's forward compatible with existing > > configs, and makes adding future versions more straightforward. > > > > GCC since ~4.8 has defaulted to this DWARF version implicitly. > > > > Suggested-by: Arvind Sankar <nivedita@alum.mit.edu> > > Suggested-by: Fangrui Song <maskray@google.com> > > Suggested-by: Nathan Chancellor <nathan@kernel.org> > > Suggested-by: Masahiro Yamada <masahiroy@kernel.org> > > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> > > One comment below: > > Reviewed-by: Nathan Chancellor <nathan@kernel.org> > > > --- > > Makefile | 5 ++--- > > lib/Kconfig.debug | 16 +++++++++++----- > > 2 files changed, 13 insertions(+), 8 deletions(-) > > > > diff --git a/Makefile b/Makefile > > index 95ab9856f357..d2b4980807e0 100644 > > --- a/Makefile > > +++ b/Makefile > > @@ -830,9 +830,8 @@ ifneq ($(LLVM_IAS),1) > > KBUILD_AFLAGS += -Wa,-gdwarf-2 > > It is probably worth a comment somewhere that assembly files will still > have DWARF v2. Such a comment should have gone on commit b8a9092330da ("Kbuild: do not emit debug info for assembly with LLVM_IAS=1") It's also more complicated than "assembly files will still have DWARF v4," due to the LLVM_IAS check. Further, such a comment would be inappropriate for patch 1 of the series, which simply changes a Kconfig option into a choice. I will add a note to the commit message of patch 2, and carry your reviewed by tag. Please nack v8 (which I plan to send imminently) if you disagree.
On Wed, Feb 3, 2021 at 4:30 PM Masahiro Yamada <masahiroy@kernel.org> wrote: > > On Thu, Feb 4, 2021 at 8:16 AM Nick Desaulniers <ndesaulniers@google.com> wrote: > > > > On Wed, Feb 3, 2021 at 2:24 PM Masahiro Yamada <masahiroy@kernel.org> wrote: > > > > > > On Sat, Jan 30, 2021 at 10:52 AM Nathan Chancellor <nathan@kernel.org> wrote: > > > > > > > > On Fri, Jan 29, 2021 at 04:44:00PM -0800, Nick Desaulniers wrote: > > > > > Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice which is > > > > > the default. Does so in a way that's forward compatible with existing > > > > > configs, and makes adding future versions more straightforward. > > > > > > > > > > GCC since ~4.8 has defaulted to this DWARF version implicitly. > > > > > > > > > > Suggested-by: Arvind Sankar <nivedita@alum.mit.edu> > > > > > Suggested-by: Fangrui Song <maskray@google.com> > > > > > Suggested-by: Nathan Chancellor <nathan@kernel.org> > > > > > Suggested-by: Masahiro Yamada <masahiroy@kernel.org> > > > > > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> > > > > > > > > One comment below: > > > > > > > > Reviewed-by: Nathan Chancellor <nathan@kernel.org> > > > > > > > > > --- > > > > > Makefile | 5 ++--- > > > > > lib/Kconfig.debug | 16 +++++++++++----- > > > > > 2 files changed, 13 insertions(+), 8 deletions(-) > > > > > > > > > > diff --git a/Makefile b/Makefile > > > > > index 95ab9856f357..d2b4980807e0 100644 > > > > > --- a/Makefile > > > > > +++ b/Makefile > > > > > @@ -830,9 +830,8 @@ ifneq ($(LLVM_IAS),1) > > > > > KBUILD_AFLAGS += -Wa,-gdwarf-2 > > > > > > > > It is probably worth a comment somewhere that assembly files will still > > > > have DWARF v2. > > > > > > I agree. > > > Please noting the reason will be helpful. > > > > Via a comment in the source, or in the commit message? > > > > > > > > Could you summarize Jakub's comment in short? > > > https://patchwork.kernel.org/project/linux-kbuild/patch/20201022012106.1875129-1-ndesaulniers@google.com/#23727667 > > > > Via a comment in the source, or in the commit message? > > > Both in the source if you can summarize it in three lines or so. > > > If you need to add more detailed explanation, > please provide it in the commit log. This information is mostly in the commit log of patch 2. I will reword it with additional info from the link you sent, but I find that such minutiae is resulting in diminishing returns on subsequent respins of the patch series. I implore you to reword v8 (which I will send) as you see fit when applying the series.
Hi Nick, On Fri, Jan 29, 2021 at 04:44:00PM -0800, Nick Desaulniers wrote: > Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice which is > the default. Does so in a way that's forward compatible with existing > configs, and makes adding future versions more straightforward. > > GCC since ~4.8 has defaulted to this DWARF version implicitly. And since GCC 11 it defaults to DWARF version 5. It would be better to set the default to the DWARF version that the compiler generates. So if the user doesn't select any version then it should default to just -g (or -gdwarf). Thanks, Mark
On Thu, Feb 4, 2021 at 2:41 AM Mark Wielaard <mark@klomp.org> wrote: > > Hi Nick, > > On Fri, Jan 29, 2021 at 04:44:00PM -0800, Nick Desaulniers wrote: > > Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice which is > > the default. Does so in a way that's forward compatible with existing > > configs, and makes adding future versions more straightforward. > > > > GCC since ~4.8 has defaulted to this DWARF version implicitly. > > And since GCC 11 it defaults to DWARF version 5. > > It would be better to set the default to the DWARF version that the > compiler generates. So if the user doesn't select any version then it > should default to just -g (or -gdwarf). I disagree. https://lore.kernel.org/lkml/CAKwvOdk0zxewEOaFuqK0aSMz3vKNzDOgmez=-Dae4+bodsSg5w@mail.gmail.com/ """ I agree that this patch takes away the compiler vendor's choice as to what the implicit default choice is for dwarf version for the kernel. (We, the Linux kernel, do so already for implicit default -std=gnuc* as well). ... But I'm going to suggest we follow the Zen of Python: explicit is better than implicit. """ We have a number of in tree and out of tree DWARF consumers that aren't ready for DWARF v5. Kernel developers need a way to disable DWARF v5 until their dependencies are deployed or more widely available. I'm happy to consider eventually moving the default DWARF version for the kernel to v5, and ideas for how to wean developers off of v4, but I don't think forcing v5 on kernel developers right now is the most delicate approach.
On Thu, 2021-02-04 at 11:18 -0800, Nick Desaulniers wrote: > On Thu, Feb 4, 2021 at 2:41 AM Mark Wielaard <mark@klomp.org> wrote: > > On Fri, Jan 29, 2021 at 04:44:00PM -0800, Nick Desaulniers wrote: > > > Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice which is > > > the default. Does so in a way that's forward compatible with existing > > > configs, and makes adding future versions more straightforward. > > > > > > GCC since ~4.8 has defaulted to this DWARF version implicitly. > > > > And since GCC 11 it defaults to DWARF version 5. > > > > It would be better to set the default to the DWARF version that the > > compiler generates. So if the user doesn't select any version then it > > should default to just -g (or -gdwarf). > > I disagree. > > https://lore.kernel.org/lkml/CAKwvOdk0zxewEOaFuqK0aSMz3vKNzDOgmez=-Dae4+bodsSg5w@mail.gmail.com/ > """ > I agree that this patch takes away the compiler vendor's choice as to > what the implicit default choice is for dwarf version for the kernel. > (We, the Linux kernel, do so already for implicit default -std=gnuc* > as well). ... > But I'm > going to suggest we follow the Zen of Python: explicit is better than > implicit. > """ > We have a number of in tree and out of tree DWARF consumers that > aren't ready for DWARF v5. Kernel developers need a way to disable > DWARF v5 until their dependencies are deployed or more widely > available. I agree with Jakub. Now that GCC has defaulted to DWARF5 all the tools have adopted to the new default version. And DWARF5 has been out for more than 4 years already. It isn't unreasonable to assume that people using GCC11 will also be using the rest of the toolchain that has moved on. Which DWARF consumers are you concerned about not being ready for GCC defaulting to DWARF5 once GCC11 is released? Thanks, Mark
On Thu, Feb 4, 2021 at 11:56 AM Mark Wielaard <mark@klomp.org> wrote: > > On Thu, 2021-02-04 at 11:18 -0800, Nick Desaulniers wrote: > > On Thu, Feb 4, 2021 at 2:41 AM Mark Wielaard <mark@klomp.org> wrote: > > > On Fri, Jan 29, 2021 at 04:44:00PM -0800, Nick Desaulniers wrote: > > > > Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice which is > > > > the default. Does so in a way that's forward compatible with existing > > > > configs, and makes adding future versions more straightforward. > > > > > > > > GCC since ~4.8 has defaulted to this DWARF version implicitly. > > > > > > And since GCC 11 it defaults to DWARF version 5. > > > > > > It would be better to set the default to the DWARF version that the > > > compiler generates. So if the user doesn't select any version then it > > > should default to just -g (or -gdwarf). > > > > I disagree. > > > > https://lore.kernel.org/lkml/CAKwvOdk0zxewEOaFuqK0aSMz3vKNzDOgmez=-Dae4+bodsSg5w@mail.gmail.com/ > > """ > > I agree that this patch takes away the compiler vendor's choice as to > > what the implicit default choice is for dwarf version for the kernel. > > (We, the Linux kernel, do so already for implicit default -std=gnuc* > > as well). ... > > But I'm > > going to suggest we follow the Zen of Python: explicit is better than > > implicit. > > """ > > We have a number of in tree and out of tree DWARF consumers that > > aren't ready for DWARF v5. Kernel developers need a way to disable > > DWARF v5 until their dependencies are deployed or more widely > > available. > > I agree with Jakub. Now that GCC has defaulted to DWARF5 all the tools > have adopted to the new default version. And DWARF5 has been out for "all of the tools" ? > more than 4 years already. It isn't unreasonable to assume that people > using GCC11 will also be using the rest of the toolchain that has moved > on. Which DWARF consumers are you concerned about not being ready for > GCC defaulting to DWARF5 once GCC11 is released? Folks who don't have top of tree pahole or binutils are the two that come to mind. I don't have specifics on out of tree consumers, but some Aarch64 extensions which had some changes to DWARF for ARMv8.3 PAC support broke some debuggers. I don't doubt a lot of work has gone into fixing many downstream projects and then when building everything from ToT that there are no issues with DWARF v5. The issue is getting upgrades into developers hands, and what to default to until then.
On Thu, 2021-02-04 at 12:04 -0800, Nick Desaulniers wrote: > On Thu, Feb 4, 2021 at 11:56 AM Mark Wielaard <mark@klomp.org> wrote: > > I agree with Jakub. Now that GCC has defaulted to DWARF5 all the > > tools > > have adopted to the new default version. And DWARF5 has been out > > for > > "all of the tools" ? I believe so yes, we did a mass-rebuild of all of Fedora a few weeks back with a GCC11 pre-release and did find some tools which weren't ready, but as far as I know all have been fixed now. I did try to coordinate with the Suse and Debian packagers too, so all the major distros should have all the necessary updates once switching to GCC11. > > more than 4 years already. It isn't unreasonable to assume that people > > using GCC11 will also be using the rest of the toolchain that has moved > > on. Which DWARF consumers are you concerned about not being ready for > > GCC defaulting to DWARF5 once GCC11 is released? > > Folks who don't have top of tree pahole or binutils are the two that > come to mind. I believe pahole just saw a 1.20 release. I am sure it will be widely available once GCC11 is released (which will still be 1 or 2 months) and people are actually using it. Or do you expect distros/people are going to upgrade to GCC11 without updating their other toolchain tools? BTW. GCC11 doesn't need top of tree binutils, it will detect the binutils capabilities (bugs) and adjust its DWARF output based on it. > I don't have specifics on out of tree consumers, but > some Aarch64 extensions which had some changes to DWARF for ARMv8.3 > PAC support broke some debuggers. It would be really helpful if you could provide some specifics. I did fix some consumers to handle the PAC operands in CFI last year, but I don't believe that had anything to do with the default DWARF version, just with dealing with DW_CFA_AARCH64_negate_ra_state. > I don't doubt a lot of work has gone into fixing many downstream > projects and then when building everything from ToT that there are no > issues with DWARF v5. The issue is getting upgrades into developers > hands, and what to default to until then. I would suggest you simply default to what you already do when the compiler is given -g. Just like you do already for the implicit default -std=gnuc*. Once GCC11 is actually released and people upgrade their toolchain to use it the tools will be ready and in developers hands. Cheers, Mark
On Thu, Feb 4, 2021 at 12:28 PM Mark Wielaard <mark@klomp.org> wrote: > > On Thu, 2021-02-04 at 12:04 -0800, Nick Desaulniers wrote: > > On Thu, Feb 4, 2021 at 11:56 AM Mark Wielaard <mark@klomp.org> wrote: > > > I agree with Jakub. Now that GCC has defaulted to DWARF5 all the > > > tools > > > have adopted to the new default version. And DWARF5 has been out > > > for > > > > "all of the tools" ? > > I believe so yes, we did a mass-rebuild of all of Fedora a few weeks > back with a GCC11 pre-release and did find some tools which weren't > ready, but as far as I know all have been fixed now. I did try to Congrats, I'm sure that was _a lot_ of work. Our toolchain folks have been pouring a lot of effort over getting our internal code all moved over, and it doesn't look like it's been easy from my perspective. > coordinate with the Suse and Debian packagers too, so all the major > distros should have all the necessary updates once switching to GCC11. That's great for users of the next Fedora release who can and will upgrade, but I wouldn't assume kernel developers can, or will (or are even using those distros). Most recently, there was discussion on the list about upgrading the minimally required version of GCC for building the kernel to GCC 5.1; we still had developers complain about abandoning GCC 4.9. And Guenter shared with me a list of architectures he tests with where he cannot upgrade the version of GCC in order to keep building them. https://github.com/groeck/linux-build-test/blob/master/bin/stable-build-arch.sh (I hope someone sent bug reports for those) My intent is very much to allow for users of toolchains that have not switched the implicit default (such as all of the supported versions of GCC that have been released ie. up to GCC 10.2, and Clang; so all toolchains the kernel still supports) to enjoy the size saving of DWARF v5, and find what other tooling needs to be updated. > > > more than 4 years already. It isn't unreasonable to assume that people > > > using GCC11 will also be using the rest of the toolchain that has moved > > > on. Which DWARF consumers are you concerned about not being ready for > > > GCC defaulting to DWARF5 once GCC11 is released? > > > > Folks who don't have top of tree pahole or binutils are the two that > > come to mind. > > I believe pahole just saw a 1.20 release. I am sure it will be widely > available once GCC11 is released (which will still be 1 or 2 months) > and people are actually using it. Or do you expect distros/people are > going to upgrade to GCC11 without updating their other toolchain tools? Does no one test linux kernel builds with top of tree GCC built from source? Or does that require "updating their other toolchain tools?" If I build ToT GCC from source, do I need to do the same for binutils-gdb in order to build the kernel? Pretty sure I don't. https://bugzilla.redhat.com/show_bug.cgi?id=1922707 and https://bugzilla.redhat.com/show_bug.cgi?id=1922698 look like user reports to me, but hopefully some CI system reported earlier that builds of the Linux kernel with GCC 11 pre release would produce the warnings from those bug report. Otherwise it looks like evidence that users "are going to upgrade to GCC11 without updating their other toolchain tools." In the case of pahole, they could not, because fixes were not yet written. "Just upgrade" doesn't work if there's no fix yet upstream. (pahole is reported fixed for that specific issue, FWIW). > BTW. GCC11 doesn't need top of tree binutils, it will detect the > binutils capabilities (bugs) and adjust its DWARF output based on it. Yes, I saw https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=6923255e35a3d54f2083ad0f67edebb3f1b86506 and https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=1aeb7d7d67d167297ca2f4a97ef20f68e7546b4c. It's nice that GCC can tightly couple to a version of binutils. Clang without its integrated assembler can make no such assumptions about which assembler the user will prefer to use instead at runtime, and without binutils 2.35.1 being widely available as we all would like, leads to issues shipping DWARF v5 by default. > > I don't have specifics on out of tree consumers, but > > some Aarch64 extensions which had some changes to DWARF for ARMv8.3 > > PAC support broke some debuggers. > > It would be really helpful if you could provide some specifics. I did > fix some consumers to handle the PAC operands in CFI last year, but I > don't believe that had anything to do with the default DWARF version, > just with dealing with DW_CFA_AARCH64_negate_ra_state. Yep, that's the one. I suspect that the same out of tree consumers of DWARF that did not see that coming will similarly be stumped when presented with DWARF v5, but it's hypothetical, so not much of an argument I'll admit. I just wouldn't bet that an upgrade to DWARF v5 will be painless at this point in time, as evidenced by how much blood has been poured into finding what tools out there were broken and needed to be fixed. I also recognize we can't drag our heels forever over this. It's not the intent of the patch series to do so. > > I don't doubt a lot of work has gone into fixing many downstream > > projects and then when building everything from ToT that there are no > > issues with DWARF v5. The issue is getting upgrades into developers > > hands, and what to default to until then. > > I would suggest you simply default to what you already do when the > compiler is given -g. Just like you do already for the implicit default > -std=gnuc*. Once GCC11 is actually released and people upgrade their > toolchain to use it the tools will be ready and in developers hands. Hmmm...thinking about this more over lunch. I think the linker script additions for the new DWARF v5 sections are most important. There's no need to hold those hostage over what we do with the configs. They are orthogonal, and can go in first. There's already one section in the linker script already related to v5, and we know that DWARF v5 is coming, so there's no reason not to just add them. That would resolve https://bugzilla.redhat.com/show_bug.cgi?id=1922707. For configs, today the kernel has an explicit opt in for DWARF v4 called CONFIG_DEBUG_INFO_DWARF4. When it was introduced, I suspect DWARF v2 (or v3) was the default, so it let kernel developers opt in to newer versions (say, for testing) than what was the implicit default version. With the advent of DWARF v5 and toolchains moving to producing that by default, maybe there is still a place for CONFIG_DEBUG_INFO_DWARF4, but as a way to opt in to older versions. Your installed version of pahole or binutils is too old for DWARF v5? Great, CONFIG_DEBUG_INFO_DWARF4 is your friend until you can update your tools. CONFIG_DEBUG_INFO_DWARF4 becomes "opt backwards" rather than it's original "opt forwards" (if that makes sense). One small issue I have with that is that it doesn't help users of GCC 5 through 10.2 (or clang) opt into DWARF v5 (to test, or for the size savings) similar to the original intent of CONFIG_DEBUG_INFO_DWARF4. That was my intent with this series; to opt into new versions so we can begin testing them and finding kinks in other tools. It was not intentionally to hold back compilers once they upgrade their default versions, even if I still think explicit is better than implicit. And it would be nice to have the framework to continue to do so for v6 someday. So opting in to v4 explicitly provides a "pressure relief valve" for developers that don't have the latest tools. Opting into v5 explicitly allows for testing of those tools with the latest DWARF version (and size savings that are worthwhile) for folks with older GCC and Clang. What are your thoughts on this? What if I modified the config to have 3 options: 1. (default) CONFIG_DEBUG_INFO_DWARF_VERSION_DONT_CARE (I'm open to a better color for the bikeshed). Does not set a -gdwarf-*. This is the status quo today when you build the kernel. You allow toolchain developers to decide your fate. If that makes you unhappy, congrats, you have the below options. 2. CONFIG_DEBUG_INFO_DWARF4 (the ship has already sailed on the name here, sorry) Explicitly sets -gdwarf-4. Use this if you have a good reason not to upgrade to the recommended -gdwarf-5. We might disappear this option soon. (already exists in tree today, and doesn't need to be removed; instead repurposed) 3. CONFIG_DEBUG_INFO_DWARF5 Explicitly sets -gdwarf-5. Use this if you would like to opt into a newer version than what's provided by default. Provides significant size savings over DWARF v4. Avoid if you have broken shit you can't upgrade for whatever reason? Then I think everyone can be happy? Toolchain vendors can to continue pursuing updates of implicit default dwarf version (aggressive, but necessary force adoption, perhaps), BTF/pahole/binutils users can downgrade until they have the necessary updates, and developers with older GCC and clang can opt in new features without having to upgrade the toolchain's implicit default (since that requires significant work for other codebases, but is making progress) and still realize the binary size savings. -- Thanks, ~Nick Desaulniers
On 2021-02-04, Nick Desaulniers wrote: >On Thu, Feb 4, 2021 at 12:28 PM Mark Wielaard <mark@klomp.org> wrote: >> >> On Thu, 2021-02-04 at 12:04 -0800, Nick Desaulniers wrote: >> > On Thu, Feb 4, 2021 at 11:56 AM Mark Wielaard <mark@klomp.org> wrote: >> > > I agree with Jakub. Now that GCC has defaulted to DWARF5 all the >> > > tools >> > > have adopted to the new default version. And DWARF5 has been out >> > > for >> > >> > "all of the tools" ? >> >> I believe so yes, we did a mass-rebuild of all of Fedora a few weeks >> back with a GCC11 pre-release and did find some tools which weren't >> ready, but as far as I know all have been fixed now. I did try to > >Congrats, I'm sure that was _a lot_ of work. Our toolchain folks have >been pouring a lot of effort over getting our internal code all moved >over, and it doesn't look like it's been easy from my perspective. > >> coordinate with the Suse and Debian packagers too, so all the major >> distros should have all the necessary updates once switching to GCC11. > >That's great for users of the next Fedora release who can and will >upgrade, but I wouldn't assume kernel developers can, or will (or are >even using those distros). > >Most recently, there was discussion on the list about upgrading the >minimally required version of GCC for building the kernel to GCC 5.1; >we still had developers complain about abandoning GCC 4.9. And >Guenter shared with me a list of architectures he tests with where he >cannot upgrade the version of GCC in order to keep building them. >https://github.com/groeck/linux-build-test/blob/master/bin/stable-build-arch.sh >(I hope someone sent bug reports for those) > >My intent is very much to allow for users of toolchains that have not >switched the implicit default (such as all of the supported versions >of GCC that have been released ie. up to GCC 10.2, and Clang; so all >toolchains the kernel still supports) to enjoy the size saving of >DWARF v5, and find what other tooling needs to be updated. > >> > > more than 4 years already. It isn't unreasonable to assume that people >> > > using GCC11 will also be using the rest of the toolchain that has moved >> > > on. Which DWARF consumers are you concerned about not being ready for >> > > GCC defaulting to DWARF5 once GCC11 is released? >> > >> > Folks who don't have top of tree pahole or binutils are the two that >> > come to mind. >> >> I believe pahole just saw a 1.20 release. I am sure it will be widely >> available once GCC11 is released (which will still be 1 or 2 months) >> and people are actually using it. Or do you expect distros/people are >> going to upgrade to GCC11 without updating their other toolchain tools? > >Does no one test linux kernel builds with top of tree GCC built from >source? Or does that require "updating their other toolchain tools?" >If I build ToT GCC from source, do I need to do the same for >binutils-gdb in order to build the kernel? Pretty sure I don't. > >https://bugzilla.redhat.com/show_bug.cgi?id=1922707 and >https://bugzilla.redhat.com/show_bug.cgi?id=1922698 look like user >reports to me, but hopefully some CI system reported earlier that >builds of the Linux kernel with GCC 11 pre release would produce the >warnings from those bug report. Otherwise it looks like evidence that >users "are going to upgrade to GCC11 without updating their other >toolchain tools." In the case of pahole, they could not, because >fixes were not yet written. "Just upgrade" doesn't work if there's no >fix yet upstream. (pahole is reported fixed for that specific issue, >FWIW). > >> BTW. GCC11 doesn't need top of tree binutils, it will detect the >> binutils capabilities (bugs) and adjust its DWARF output based on it. > >Yes, I saw https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=6923255e35a3d54f2083ad0f67edebb3f1b86506 >and https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=1aeb7d7d67d167297ca2f4a97ef20f68e7546b4c. >It's nice that GCC can tightly couple to a version of binutils. Clang >without its integrated assembler can make no such assumptions about >which assembler the user will prefer to use instead at runtime, and >without binutils 2.35.1 being widely available as we all would like, >leads to issues shipping DWARF v5 by default. > >> > I don't have specifics on out of tree consumers, but >> > some Aarch64 extensions which had some changes to DWARF for ARMv8.3 >> > PAC support broke some debuggers. >> >> It would be really helpful if you could provide some specifics. I did >> fix some consumers to handle the PAC operands in CFI last year, but I >> don't believe that had anything to do with the default DWARF version, >> just with dealing with DW_CFA_AARCH64_negate_ra_state. > >Yep, that's the one. I suspect that the same out of tree consumers of >DWARF that did not see that coming will similarly be stumped when >presented with DWARF v5, but it's hypothetical, so not much of an >argument I'll admit. I just wouldn't bet that an upgrade to DWARF v5 >will be painless at this point in time, as evidenced by how much blood >has been poured into finding what tools out there were broken and >needed to be fixed. I also recognize we can't drag our heels forever >over this. It's not the intent of the patch series to do so. > >> > I don't doubt a lot of work has gone into fixing many downstream >> > projects and then when building everything from ToT that there are no >> > issues with DWARF v5. The issue is getting upgrades into developers >> > hands, and what to default to until then. >> >> I would suggest you simply default to what you already do when the >> compiler is given -g. Just like you do already for the implicit default >> -std=gnuc*. Once GCC11 is actually released and people upgrade their >> toolchain to use it the tools will be ready and in developers hands. > >Hmmm...thinking about this more over lunch. > >I think the linker script additions for the new DWARF v5 sections are >most important. There's no need to hold those hostage over what we do >with the configs. They are orthogonal, and can go in first. There's >already one section in the linker script already related to v5, and we >know that DWARF v5 is coming, so there's no reason not to just add >them. That would resolve >https://bugzilla.redhat.com/show_bug.cgi?id=1922707. > >For configs, today the kernel has an explicit opt in for DWARF v4 >called CONFIG_DEBUG_INFO_DWARF4. When it was introduced, I suspect >DWARF v2 (or v3) was the default, so it let kernel developers opt in >to newer versions (say, for testing) than what was the implicit >default version. With the advent of DWARF v5 and toolchains moving to >producing that by default, maybe there is still a place for >CONFIG_DEBUG_INFO_DWARF4, but as a way to opt in to older versions. >Your installed version of pahole or binutils is too old for DWARF v5? >Great, CONFIG_DEBUG_INFO_DWARF4 is your friend until you can update >your tools. CONFIG_DEBUG_INFO_DWARF4 becomes "opt backwards" rather >than it's original "opt forwards" (if that makes sense). > >One small issue I have with that is that it doesn't help users of GCC >5 through 10.2 (or clang) opt into DWARF v5 (to test, or for the size >savings) similar to the original intent of CONFIG_DEBUG_INFO_DWARF4. >That was my intent with this series; to opt into new versions so we >can begin testing them and finding kinks in other tools. It was not >intentionally to hold back compilers once they upgrade their default >versions, even if I still think explicit is better than implicit. And >it would be nice to have the framework to continue to do so for v6 >someday. > >So opting in to v4 explicitly provides a "pressure relief valve" for >developers that don't have the latest tools. Opting into v5 >explicitly allows for testing of those tools with the latest DWARF >version (and size savings that are worthwhile) for folks with older >GCC and Clang. > >What are your thoughts on this? > >What if I modified the config to have 3 options: >1. (default) CONFIG_DEBUG_INFO_DWARF_VERSION_DONT_CARE (I'm open to a >better color for the bikeshed). Does not set a -gdwarf-*. This is the >status quo today when you build the kernel. You allow toolchain >developers to decide your fate. If that makes you unhappy, congrats, >you have the below options. >2. CONFIG_DEBUG_INFO_DWARF4 (the ship has already sailed on the name >here, sorry) Explicitly sets -gdwarf-4. Use this if you have a good >reason not to upgrade to the recommended -gdwarf-5. We might >disappear this option soon. (already exists in tree today, and doesn't >need to be removed; instead repurposed) >3. CONFIG_DEBUG_INFO_DWARF5 Explicitly sets -gdwarf-5. Use this if >you would like to opt into a newer version than what's provided by >default. Provides significant size savings over DWARF v4. Avoid if >you have broken shit you can't upgrade for whatever reason? > >Then I think everyone can be happy? Toolchain vendors can to continue >pursuing updates of implicit default dwarf version (aggressive, but >necessary force adoption, perhaps), BTF/pahole/binutils users can >downgrade until they have the necessary updates, and developers with >older GCC and clang can opt in new features without having to upgrade >the toolchain's implicit default (since that requires significant work >for other codebases, but is making progress) and still realize the >binary size savings. The points raised by Nick are very reasonable. A few colleagues and I are involved in upgrading our internal systems to be DWARF v5 ready. It required to fix a dozen of things. Externally I watch binutils mailing list closely and in recent months DWARF fixes are quite regular. (Mark and H.J. contributed lot of fixes. Big thanks!) For example, objdump -S did not display source for gcc-11 compiled object files. It was just fixed a few days ago https://sourceware.org/bugzilla/show_bug.cgi?id=27231 , which is included in binutils 2.35.2 (https://sourceware.org/pipermail/binutils/2021-January/115150.html). As we all know DWARF v5 has huge size savings and we want to make it the default, but today is probably a bit early. (Note about Clang and binutils: I added -fbinutils-version= to Clang 12. If there are workaround needs we can use that option.)
Hi Nick, On Thu, 2021-02-04 at 14:06 -0800, Nick Desaulniers wrote: > On Thu, Feb 4, 2021 at 12:28 PM Mark Wielaard <mark@klomp.org> wrote: > > I believe so yes, we did a mass-rebuild of all of Fedora a few weeks > > back with a GCC11 pre-release and did find some tools which weren't > > ready, but as far as I know all have been fixed now. I did try to > > Congrats, I'm sure that was _a lot_ of work. Our toolchain folks have > been pouring a lot of effort over getting our internal code all moved > over, and it doesn't look like it's been easy from my perspective. > > > coordinate with the Suse and Debian packagers too, so all the major > > distros should have all the necessary updates once switching to GCC11. > > That's great for users of the next Fedora release who can and will > upgrade, but I wouldn't assume kernel developers can, or will (or are > even using those distros). And that is fine, then they will probably also not run GCC11 yet when it gets released. GCC11 (and Fedora34) will not be released for another 2 months. By then I hope all issues found have made it into upstream releases. > My intent is very much to allow for users of toolchains that have not > switched the implicit default (such as all of the supported versions > of GCC that have been released ie. up to GCC 10.2, and Clang; so all > toolchains the kernel still supports) to enjoy the size saving of > DWARF v5, and find what other tooling needs to be updated. Which seems a good thing. No complaints about making it easier to experiment with DWARFv5 for such older compiler releases. > Does no one test linux kernel builds with top of tree GCC built from > source? Fedora rawhide (f34) builds rc kernels against GCC11 ToT now: https://koji.fedoraproject.org/koji/packageinfo?packageID=8 > Or does that require "updating their other toolchain tools?" > If I build ToT GCC from source, do I need to do the same for > binutils-gdb in order to build the kernel? Pretty sure I don't. It isn't required, but then GCC won't produce full DWARF5 (e.g. line tables are then still emitted as DWARF4). > https://bugzilla.redhat.com/show_bug.cgi?id=1922707 and > https://bugzilla.redhat.com/show_bug.cgi?id=1922698 look like user > reports to me, but hopefully some CI system reported earlier that > builds of the Linux kernel with GCC 11 pre release would produce the > warnings from those bug report. Otherwise it looks like evidence that > users "are going to upgrade to GCC11 without updating their other > toolchain tools." In the case of pahole, they could not, because > fixes were not yet written. "Just upgrade" doesn't work if there's no > fix yet upstream. (pahole is reported fixed for that specific issue, > FWIW). So those are users that are participating in the Fedora rawhide experiment with the GC11-prerelease. That are precisely the bug reports we need to make sure we caught all issues before the final release. I don't think that is any different from users trying out rc kernels and finding issues that are then fixed. > > > I don't doubt a lot of work has gone into fixing many downstream > > > projects and then when building everything from ToT that there are no > > > issues with DWARF v5. The issue is getting upgrades into developers > > > hands, and what to default to until then. > > > > I would suggest you simply default to what you already do when the > > compiler is given -g. Just like you do already for the implicit default > > -std=gnuc*. Once GCC11 is actually released and people upgrade their > > toolchain to use it the tools will be ready and in developers hands. > > Hmmm...thinking about this more over lunch. > > I think the linker script additions for the new DWARF v5 sections are > most important. There's no need to hold those hostage over what we do > with the configs. They are orthogonal, and can go in first. There's > already one section in the linker script already related to v5, and we > know that DWARF v5 is coming, so there's no reason not to just add > them. That would resolve > https://bugzilla.redhat.com/show_bug.cgi?id=1922707. Yes, that patch seems fine without any of the other config changes. It would be nice to see that go in. > For configs, today the kernel has an explicit opt in for DWARF v4 > called CONFIG_DEBUG_INFO_DWARF4. When it was introduced, I suspect > DWARF v2 (or v3) was the default, so it let kernel developers opt in > to newer versions (say, for testing) than what was the implicit > default version. With the advent of DWARF v5 and toolchains moving to > producing that by default, maybe there is still a place for > CONFIG_DEBUG_INFO_DWARF4, but as a way to opt in to older versions. > Your installed version of pahole or binutils is too old for DWARF v5? > Great, CONFIG_DEBUG_INFO_DWARF4 is your friend until you can update > your tools. CONFIG_DEBUG_INFO_DWARF4 becomes "opt backwards" rather > than it's original "opt forwards" (if that makes sense). > > One small issue I have with that is that it doesn't help users of GCC > 5 through 10.2 (or clang) opt into DWARF v5 (to test, or for the size > savings) similar to the original intent of CONFIG_DEBUG_INFO_DWARF4. > That was my intent with this series; to opt into new versions so we > can begin testing them and finding kinks in other tools. It was not > intentionally to hold back compilers once they upgrade their default > versions, even if I still think explicit is better than implicit. And > it would be nice to have the framework to continue to do so for v6 > someday. > > So opting in to v4 explicitly provides a "pressure relief valve" for > developers that don't have the latest tools. Opting into v5 > explicitly allows for testing of those tools with the latest DWARF > version (and size savings that are worthwhile) for folks with older > GCC and Clang. > > What are your thoughts on this? Yes, being able to opt into DWARF4 if you have some newer tools like GCC11 that already default to version 5, but not others, like an old pahole release that doesn't handle version 5 yet would be nice to have. But I don't think that is a very likely scenario given that pahole (and binutils) already got a new releases and GCC11 won't be released for a couple of months yet. Likewise having the option to opt into DWARF5 even though some of the tools don't default to (or fully support) DWARF5 yet seems a good idea. Just for users to see whether that works with all their tools. > What if I modified the config to have 3 options: > 1. (default) CONFIG_DEBUG_INFO_DWARF_VERSION_DONT_CARE (I'm open to a > better color for the bikeshed). Does not set a -gdwarf-*. This is the > status quo today when you build the kernel. You allow toolchain > developers to decide your fate. If that makes you unhappy, congrats, > you have the below options. My color of the bikeshed would be CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT. > 2. CONFIG_DEBUG_INFO_DWARF4 (the ship has already sailed on the name > here, sorry) Explicitly sets -gdwarf-4. Use this if you have a good > reason not to upgrade to the recommended -gdwarf-5. We might > disappear this option soon. (already exists in tree today, and doesn't > need to be removed; instead repurposed) > 3. CONFIG_DEBUG_INFO_DWARF5 Explicitly sets -gdwarf-5. Use this if > you would like to opt into a newer version than what's provided by > default. Provides significant size savings over DWARF v4. Avoid if > you have broken shit you can't upgrade for whatever reason? > > Then I think everyone can be happy? Toolchain vendors can to continue > pursuing updates of implicit default dwarf version (aggressive, but > necessary force adoption, perhaps), BTF/pahole/binutils users can > downgrade until they have the necessary updates, and developers with > older GCC and clang can opt in new features without having to upgrade > the toolchain's implicit default (since that requires significant work > for other codebases, but is making progress) and still realize the > binary size savings. That would certainly make me happy :) Thanks, Mark
On Fri, Feb 5, 2021 at 4:49 AM Mark Wielaard <mark@klomp.org> wrote: > > Hi Nick, > > On Thu, 2021-02-04 at 14:06 -0800, Nick Desaulniers wrote: > > On Thu, Feb 4, 2021 at 12:28 PM Mark Wielaard <mark@klomp.org> wrote: > > > I believe so yes, we did a mass-rebuild of all of Fedora a few weeks > > > back with a GCC11 pre-release and did find some tools which weren't > > > ready, but as far as I know all have been fixed now. I did try to I guess I'm curious whether https://bugzilla.redhat.com/show_bug.cgi?id=1922707 came up during the mass rebuild of all of Fedora a few weeks ago? Assuming the Linux kernel was part of that test, those warnings would have been both new and obviously related to changing the implicit default DWARF version. It's possible it was a kernel build without debug info enabled or a kernel version old enough to not have that linker warning enabled, I suppose. It might be good to check though, that way changes to GCC that impact the kernel are caught even sooner or ASAP. New diagnostics added every compiler release come to mind in particular.
Hi Nick, On Fri, Feb 05, 2021 at 01:18:11PM -0800, Nick Desaulniers wrote: > On Fri, Feb 5, 2021 at 4:49 AM Mark Wielaard <mark@klomp.org> wrote: > I guess I'm curious whether > https://bugzilla.redhat.com/show_bug.cgi?id=1922707 came up during the > mass rebuild of all of Fedora a few weeks ago? Assuming the Linux > kernel was part of that test, those warnings would have been both new > and obviously related to changing the implicit default DWARF version. Yes, looking at the build.log that warning was also present. But the dwarves pahole update to process DWARF5 was more important. Also at first it was believed this came from the binutils ld linker scripts. Which were also updated first. Once your patch is accepted we can resolve that bug. Cheers, Mark
diff --git a/Makefile b/Makefile index 95ab9856f357..d2b4980807e0 100644 --- a/Makefile +++ b/Makefile @@ -830,9 +830,8 @@ ifneq ($(LLVM_IAS),1) KBUILD_AFLAGS += -Wa,-gdwarf-2 endif -ifdef CONFIG_DEBUG_INFO_DWARF4 -DEBUG_CFLAGS += -gdwarf-4 -endif +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4 +DEBUG_CFLAGS += -gdwarf-$(dwarf-version-y) ifdef CONFIG_DEBUG_INFO_REDUCED DEBUG_CFLAGS += $(call cc-option, -femit-struct-debug-baseonly) \ diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index e906ea906cb7..94c1a7ed6306 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -256,13 +256,19 @@ config DEBUG_INFO_SPLIT to know about the .dwo files and include them. Incompatible with older versions of ccache. +choice + prompt "DWARF version" + help + Which version of DWARF debug info to emit. + config DEBUG_INFO_DWARF4 - bool "Generate dwarf4 debuginfo" + bool "Generate DWARF Version 4 debuginfo" help - Generate dwarf4 debug info. This requires recent versions - of gcc and gdb. It makes the debug information larger. - But it significantly improves the success of resolving - variables in gdb on optimized code. + Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+. + It makes the debug information larger, but it significantly + improves the success of resolving variables in gdb on optimized code. + +endchoice # "DWARF version" config DEBUG_INFO_BTF bool "Generate BTF typeinfo"
Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice which is the default. Does so in a way that's forward compatible with existing configs, and makes adding future versions more straightforward. GCC since ~4.8 has defaulted to this DWARF version implicitly. Suggested-by: Arvind Sankar <nivedita@alum.mit.edu> Suggested-by: Fangrui Song <maskray@google.com> Suggested-by: Nathan Chancellor <nathan@kernel.org> Suggested-by: Masahiro Yamada <masahiroy@kernel.org> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> --- Makefile | 5 ++--- lib/Kconfig.debug | 16 +++++++++++----- 2 files changed, 13 insertions(+), 8 deletions(-)