linux-kernel - Re: [PATCH v3 2/3] riscv: vdso: Disable LTO for the vDSO

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Message-ID: <20250611184106.GB2192624@ax162>
Date: Wed, 11 Jun 2025 11:41:06 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Andy Lutomirski <luto@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vincenzo Frascino <vincenzo.frascino@....com>,
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: Re: [PATCH v3 2/3] riscv: vdso: Disable LTO for the vDSO

On Wed, Jun 11, 2025 at 11:22:13AM +0200, Thomas Weißschuh wrote:
> All the other architectures supporting LTO (x86, arm64, loongarch) do not
> use it for the vDSO.
> 
> Its is problematic for some upcoming compile-time validation of the
> generated object code.
> The LTO object files do not contain the necessary relocation information
> and -flto-fat-objects is not compatible with clang < 16.
> 
> For consistency and to enable the mentioned compile-time checks,
> disable LTO for the vDSO.
> The vDSO heavily uses __always_inline anyways.
> 
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>

Yes, I would agree this has little actual value for the vDSO.

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
>  arch/riscv/kernel/vdso/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
> index c19c3c76f7c9f6b2f7523a59269de3b683656323..9f1bf5bae9bd473e43d9fd3022e9e1a185128b73 100644
> --- a/arch/riscv/kernel/vdso/Makefile
> +++ b/arch/riscv/kernel/vdso/Makefile
> @@ -49,7 +49,7 @@ CPPFLAGS_vdso.lds += -DHAS_VGETTIMEOFDAY
>  endif
>  
>  # Disable -pg to prevent insert call site
> -CFLAGS_REMOVE_VDSO = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)
> +CFLAGS_REMOVE_VDSO = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_LTO)
>  CFLAGS_REMOVE_vgettimeofday.o = $(CFLAGS_REMOVE_VDSO)
>  CFLAGS_REMOVE_getrandom.o = $(CFLAGS_REMOVE_VDSO)
>  CFLAGS_REMOVE_hwprobe.o = $(CFLAGS_REMOVE_VDSO)
> 
> -- 
> 2.49.0
>