Re: [PATCH] riscv: vdso: fix build with llvm

[Conor Dooley <conor@kernel.org>]

On Tue, Nov 01, 2022 at 02:29:43AM +0800, Jisheng Zhang wrote:
> Even after commit 89fd4a1df829 ("riscv: jump_label: mark arguments as
> const to satisfy asm constraints"), building with CC_OPTIMIZE_FOR_SIZE
> + LLVM=1 can reproduce below build error:
> 
>   CC      arch/riscv/kernel/vdso/vgettimeofday.o
> In file included from <built-in>:4:
> In file included from lib/vdso/gettimeofday.c:5:
> In file included from include/vdso/datapage.h:17:
> In file included from
> include/vdso/processor.h:10:
> In file included from
> arch/riscv/include/asm/vdso/processor.h:7:
> In file included from
> include/linux/jump_label.h:112:
> arch/riscv/include/asm/jump_label.h:42:3: error:
> invalid operand fo
> r inline asm constraint 'i'
>                 "       .option push                            \n\t"
>                 ^
> 1 error generated.

Tested-by: Conor Dooley <conor.dooley@microchip.com>

> 
> I think the problem is when "-Os" is passed as CFLAGS, it's removed by
> "CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE) -Os" which is
> introduced in commit e05d57dcb8c7 ("riscv: Fixup __vdso_gettimeofday
> broke dynamic ftrace"), thus no optimization at all for vgettimeofday.c
> arm64 does remove "-Os" as well, but it forces "-O2" after removing
> "-Os".
> 
> I compared the generated vgettimeofday.o with "-O2" and "-Os",
> I think no big performance difference. So let's tell the kbuild not
> to remove "-Os" rather than follow arm64 style.
> 
> vdso related performance can be improved a lot when building kernel with
> CC_OPTIMIZE_FOR_SIZE after this commit, ("-Os" VS no optimization)
> 
> Fixes: e05d57dcb8c7 ("riscv: Fixup __vdso_gettimeofday broke dynamic ftrace")
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> ---
>  arch/riscv/kernel/vdso/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
> index f2e065671e4d..84ac0fe612e7 100644
> --- a/arch/riscv/kernel/vdso/Makefile
> +++ b/arch/riscv/kernel/vdso/Makefile
> @@ -30,7 +30,7 @@ obj-y += vdso.o
>  CPPFLAGS_vdso.lds += -P -C -U$(ARCH)
>  
>  # Disable -pg to prevent insert call site
> -CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE) -Os
> +CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE)
>  
>  # Disable profiling and instrumentation for VDSO code
>  GCOV_PROFILE := n
> -- 
> 2.37.2
>