Re: [PATCH] x86: Enable clang LTO for 32-bit as well

From: Nick Desaulniers <ndesaulniers@google.com>
To: Nathan Chancellor <nathan@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, Sami Tolvanen <samitolvanen@google.com>,
	Kees Cook <keescook@chromium.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	LKML <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH] x86: Enable clang LTO for 32-bit as well
Date: Fri, 30 Apr 2021 13:58:57 -0700	[thread overview]
Message-ID: <CAKwvOdngd5inT_sQP-PgqaMpVzfSdwoyhCcUSdS+09u=PKUEyA@mail.gmail.com> (raw)
In-Reply-To: <20210429232611.3966964-1-nathan@kernel.org>

On Thu, Apr 29, 2021 at 4:26 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> Commit b33fff07e3e3 ("x86, build: allow LTO to be selected") enabled
> support for LTO for x86_64 but 32-bit works fine as well.
>
> I tested the following config combinations:
>
> * i386_defconfig + CONFIG_LTO_CLANG_FULL=y
>
> * i386_defconfig + CONFIG_LTO_CLANG_THIN=y
>
> * ARCH=i386 allmodconfig + CONFIG_LTO_CLANG_THIN=y

For allmodconfig
HAS_LTO_CLANG cannot be selected unless
FTRACE
GCOV_KERNEL
are disabled.

I was able to run through similar tests, though I only did so for ToT LLVM.

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>

I additionally tried full LTO allmodconfig.  Full LTO allmodconfig can
be tested by disabling CONFIG_COMPILE_TEST (in addition to the above),
though I hit https://github.com/ClangBuiltLinux/linux/issues/1215 (the
xfs tests aren't building under LTO).  I think this change though is
fine, and enables another avenue to help us debug that failure.

>
> with LLVM 11.1.0, 12.0.0, and 13.0.0 from git without any build
> failures. The defconfigs boot in QEMU with no new warnings.
>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>  arch/x86/Kconfig | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 0fc82237414d..d8a919d9535e 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -97,8 +97,8 @@ config X86
>         select ARCH_SUPPORTS_DEBUG_PAGEALLOC
>         select ARCH_SUPPORTS_NUMA_BALANCING     if X86_64
>         select ARCH_SUPPORTS_KMAP_LOCAL_FORCE_MAP       if NR_CPUS <= 4096
> -       select ARCH_SUPPORTS_LTO_CLANG          if X86_64
> -       select ARCH_SUPPORTS_LTO_CLANG_THIN     if X86_64
> +       select ARCH_SUPPORTS_LTO_CLANG
> +       select ARCH_SUPPORTS_LTO_CLANG_THIN
>         select ARCH_USE_BUILTIN_BSWAP
>         select ARCH_USE_QUEUED_RWLOCKS
>         select ARCH_USE_QUEUED_SPINLOCKS
>
> base-commit: d8201efe75e13146ebde433745c7920e15593baf
> --
> 2.31.1.362.g311531c9de
>

-- 
Thanks,
~Nick Desaulniers