linux-kernel - Re: [PATCH v4 2/3] Kbuild: make DWARF version a choice

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Date:   Thu, 14 Jan 2021 08:20:30 +0100
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Jakub Jelinek <jakub@...hat.com>,
        Fangrui Song <maskray@...gle.com>,
        Caroline Tice <cmtice@...gle.com>,
        Nick Clifton <nickc@...hat.com>
Subject: Re: [PATCH v4 2/3] Kbuild: make DWARF version a choice

On Thu, Jan 14, 2021 at 12:27 AM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> Sedat,
> Thanks for testing, and congrats on https://lwn.net/Articles/839772/.
> I always appreciate you taking the time to help test my work, and
> other Clang+Linux kernel patches!
>

Hi Nick,

cool, again in the top 15 :-).

I should ask Mr. Corbet for a LWN subscription.

> On Wed, Jan 13, 2021 at 1:24 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
> >
> > On Wed, Jan 13, 2021 at 1:32 AM Nick Desaulniers
> > <ndesaulniers@...gle.com> wrote:
> > >
> > > --- a/Makefile
> > > +++ b/Makefile
> > > @@ -826,12 +826,16 @@ else
> > >  DEBUG_CFLAGS   += -g
> > >  endif
> > >
> > > -ifneq ($(LLVM_IAS),1)
> > > -KBUILD_AFLAGS  += -Wa,-gdwarf-2
> > > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
> > > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> > > +DEBUG_CFLAGS   += -gdwarf-$(dwarf-version-y)
>
> ^ DEBUG_CFLAGS are set for everyone (all toolchains) if
> CONFIG_DEBUG_INFO is defined.
>
> > > +ifneq ($(dwarf-version-y)$(LLVM_IAS),21)
>
> ^ "If not using dwarf 2 and LLVM_IAS=1", ie. CONFIG_DEBUG_INFO_DWARF5
> && CONFIG_CC_IS_GCC
>

OK, I know DWARF v2 and LLVM_IAS=1 is broken.

Looks like DWARF v5 with GCC v10.2.1 and binutils v2.35.1 is currently
(here) no good choice.

> > > +# Binutils 2.35+ required for -gdwarf-4+ support.
> > > +dwarf-aflag    := $(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y))
> > > +ifdef CONFIG_CC_IS_CLANG
>
> ^ "if clang"
>
> > > +DEBUG_CFLAGS   += $(dwarf-aflag)
> > >  endif
> >
> > Why is that "ifdef CONFIG_CC_IS_CLANG"?
>
> That's what Arvind requested on v2, IIUC:
> https://lore.kernel.org/lkml/X8psgMuL4jMjP%2FOy@rani.riverdale.lan/
>
> > When I use GCC v10.2.1 DEBUG_CFLAGS are not set.
>
> You should have -gdwarf-4 (and not -Wa,-gwarf-4) set for DEBUG_CFLAGS
> when compiling with GCC and enabling CONFIG_DEBUG_INFO_DWARF4. Can you
> please confirm? (Perhaps you may have accidentally disabled
> CONFIG_DEBUG_INFO by rerunning `make defconfig`?)
>

$ egrep 'CC_IS_|LD_IS|BTF|DWARF'
config-5.11.0-rc3-5-amd64-gcc10-llvm11 | grep ^CONFIG
CONFIG_CC_IS_GCC=y
CONFIG_LD_IS_LLD=y
CONFIG_DEBUG_INFO_DWARF4=y
CONFIG_DEBUG_INFO_BTF=y
CONFIG_DEBUG_INFO_BTF_MODULES=y

$ grep '\-Wa,-gdwarf-4' build-log_5.11.0-rc3-5-amd64-gcc10-llvm11.txt
| wc -l
156