linux-kernel - Re: [PATCH 2/2] x86/asm/entry/32: Remove unnecessary optimization in stub32

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Date:	Wed, 22 Apr 2015 09:54:24 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Denys Vlasenko <dvlasenk@...hat.com>,
	Josh Triplett <josh@...htriplett.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] x86/asm/entry/32: Remove unnecessary optimization in stub32_clone

On Wed, Apr 22, 2015 at 9:40 AM, Denys Vlasenko <dvlasenk@...hat.com> 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@...hat.com>
> CC: Linus Torvalds <torvalds@...ux-foundation.org>
> CC: Steven Rostedt <rostedt@...dmis.org>
> CC: Ingo Molnar <mingo@...nel.org>
> CC: Borislav Petkov <bp@...en8.de>
> CC: "H. Peter Anvin" <hpa@...or.com>
> CC: Andy Lutomirski <luto@...capital.net>
> CC: Oleg Nesterov <oleg@...hat.com>
> CC: Frederic Weisbecker <fweisbec@...il.com>
> CC: Alexei Starovoitov <ast@...mgrid.com>
> CC: Will Drewry <wad@...omium.org>
> CC: Kees Cook <keescook@...omium.org>
> CC: x86@...nel.org
> CC: linux-kernel@...r.kernel.org
> ---
>  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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