Re: [PATCH 2/2] x86/asm/entry/32: Remove unnecessary optimization in stub32_clone

[Andy Lutomirski]

On Wed, Apr 22, 2015 at 9:40 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> Really swap arguments #4 and #5 in stub32_clone instead of "optimizing"
> it into a move.
>
> Yes, tls_val is currently unused. Yes, on some CPUs XCHG is a little bit
> more expensive than MOV. But a cycle or two on an expensive syscall like
> clone() is way below noise floor, and obfuscation of logic introduced
> by this optimization is simply not worth it.

Ditto re: Josh's patch.

--Andy

>
> Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
> CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
> CC: Ingo Molnar <mingo@xxxxxxxxxx>
> CC: Borislav Petkov <bp@xxxxxxxxx>
> CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
> CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> CC: Oleg Nesterov <oleg@xxxxxxxxxx>
> CC: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> CC: Alexei Starovoitov <ast@xxxxxxxxxxxx>
> CC: Will Drewry <wad@xxxxxxxxxxxx>
> CC: Kees Cook <keescook@xxxxxxxxxxxx>
> CC: x86@xxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  arch/x86/ia32/ia32entry.S | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
> index 8e72256..0c302d0 100644
> --- a/arch/x86/ia32/ia32entry.S
> +++ b/arch/x86/ia32/ia32entry.S
> @@ -567,11 +567,9 @@ GLOBAL(stub32_clone)
>          * 32-bit clone API is clone(..., int tls_val, int *child_tidptr).
>          * 64-bit clone API is clone(..., int *child_tidptr, int tls_val).
>          * Native 64-bit kernel's sys_clone() implements the latter.
> -        * We need to swap args here. But since tls_val is in fact ignored
> -        * by sys_clone(), we can get away with an assignment
> -        * (arg4 = arg5) instead of a full swap:
> +        * We need to swap args here:
>          */
> -       mov     %r8, %rcx
> +       xchg    %r8, %rcx
>         jmp     ia32_ptregs_common
>
>         ALIGN
> --
> 1.8.1.4
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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