[PATCH 0/2] riscv: vdso: Do not use LTO for the vDSO

[PATCH 0/2] riscv: vdso: Do not use LTO for the vDSO

[Thomas Weißschuh]

With LTO enabled the compiler assumes that the vDSO functions are not
used and optimizes them away completely.

Disable LTO for the vDSO, as these functions are hand-optimized anyways.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
Thomas Weißschuh (2):
      riscv: vdso: Do not use LTO for the vDSO
      riscv: vdso: Simplify cflags remove logic

 arch/riscv/kernel/vdso/Makefile | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)
---
base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
change-id: 20260630-riscv-vdso-lto-2a5c6f021162

Best regards,
--  
Thomas Weißschuh (Schneider Electric) <thomas.weissschuh@linutronix.de>

[PATCH 1/2] riscv: vdso: Do not use LTO for the vDSO

[Thomas Weißschuh]

With LTO enabled the compiler assumes that the vDSO functions are not
used and optimizes them away completely. Currently this happens to
__vdso_clock_getres(), __vdso_clock_gettime(), __vdso_getrandom(),
__vdso_gettimeofday() and __vdso_riscv_hwprobe().

Disable LTO for the vDSO, as these functions are hand-optimized anyways.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202606301855.WvkSC4kD-lkp@intel.com/
Fixes: 021d23428bdb ("RISC-V: build: Allow LTO to be selected")
Cc: stable@vger.kernel.org
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
 arch/riscv/kernel/vdso/Makefile | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
index a842dc034571..43ee881f6c6f 100644
--- a/arch/riscv/kernel/vdso/Makefile
+++ b/arch/riscv/kernel/vdso/Makefile
@@ -69,9 +69,9 @@ CPPFLAGS_$(vdso_lds) += -DHAS_VGETTIMEOFDAY
 endif
 
 # Disable -pg to prevent insert call site
-CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)
-CFLAGS_REMOVE_getrandom.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)
-CFLAGS_REMOVE_hwprobe.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)
+CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_LTO)
+CFLAGS_REMOVE_getrandom.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_LTO)
+CFLAGS_REMOVE_hwprobe.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_LTO)
 
 # Force dependency
 $(obj)/$(vdso_o): $(obj)/$(vdso_so)

-- 
2.55.0

[PATCH 2/2] riscv: vdso: Simplify cflags remove logic

[Thomas Weißschuh]

Listing all flags for each object file is tedious and error-prone.

Replace it with a simpler solution.

Link: https://lore.kernel.org/all/20260630135316-f26f0e0f-c08c-4d4d-9963-10f9985a7689@linutronix.de/
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
 arch/riscv/kernel/vdso/Makefile | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
index 43ee881f6c6f..8dbf2532a573 100644
--- a/arch/riscv/kernel/vdso/Makefile
+++ b/arch/riscv/kernel/vdso/Makefile
@@ -41,6 +41,8 @@ ccflags-y += $(CFI_FULL)
 asflags-y += $(KBUILD_BASE_ISA)$(CFI_MARCH)
 asflags-y += $(CFI_FULL)
 
+ccflags-remove-y += $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_LTO)
+
 ifneq ($(c-gettimeofday-y),)
   CFLAGS_vgettimeofday.o += -fPIC -include $(c-gettimeofday-y)
 endif
@@ -68,11 +70,6 @@ ifneq ($(filter vgettimeofday, $(vdso-syms)),)
 CPPFLAGS_$(vdso_lds) += -DHAS_VGETTIMEOFDAY
 endif
 
-# Disable -pg to prevent insert call site
-CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_LTO)
-CFLAGS_REMOVE_getrandom.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_LTO)
-CFLAGS_REMOVE_hwprobe.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_LTO)
-
 # Force dependency
 $(obj)/$(vdso_o): $(obj)/$(vdso_so)
 

-- 
2.55.0

Re: [PATCH 0/2] riscv: vdso: Do not use LTO for the vDSO

[Nam Cao]

Thomas Weißschuh <thomas.weissschuh@linutronix.de> writes:
> With LTO enabled the compiler assumes that the vDSO functions are not
> used and optimizes them away completely.
>
> Disable LTO for the vDSO, as these functions are hand-optimized anyways.
>
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>

Reviewed-by: Nam Cao <namcao@linutronix.de>

Re: [PATCH 1/2] riscv: vdso: Do not use LTO for the vDSO

[Nathan Chancellor]

On Wed, Jul 01, 2026 at 11:21:22AM +0200, Thomas Weißschuh wrote:
> With LTO enabled the compiler assumes that the vDSO functions are not
> used and optimizes them away completely. Currently this happens to
> __vdso_clock_getres(), __vdso_clock_gettime(), __vdso_getrandom(),
> __vdso_gettimeofday() and __vdso_riscv_hwprobe().
> 
> Disable LTO for the vDSO, as these functions are hand-optimized anyways.
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202606301855.WvkSC4kD-lkp@intel.com/

While this change seems correct, is this really the fix for that report?
It seems like that error happens in clang but I would expect this sort
of issue to only appear once LTO has run through ld.lld?

> Fixes: 021d23428bdb ("RISC-V: build: Allow LTO to be selected")
> Cc: stable@vger.kernel.org
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
> ---
>  arch/riscv/kernel/vdso/Makefile | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
> index a842dc034571..43ee881f6c6f 100644
> --- a/arch/riscv/kernel/vdso/Makefile
> +++ b/arch/riscv/kernel/vdso/Makefile
> @@ -69,9 +69,9 @@ CPPFLAGS_$(vdso_lds) += -DHAS_VGETTIMEOFDAY
>  endif
>  
>  # Disable -pg to prevent insert call site
> -CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)
> -CFLAGS_REMOVE_getrandom.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)
> -CFLAGS_REMOVE_hwprobe.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)
> +CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_LTO)
> +CFLAGS_REMOVE_getrandom.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_LTO)
> +CFLAGS_REMOVE_hwprobe.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_LTO)
>  
>  # Force dependency
>  $(obj)/$(vdso_o): $(obj)/$(vdso_so)
> 
> -- 
> 2.55.0
> 

-- 
Cheers,
Nathan