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SubjectRe: [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@redhat.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@redhat.com>
> CC: Linus Torvalds <torvalds@linux-foundation.org>
> CC: Steven Rostedt <rostedt@goodmis.org>
> CC: Ingo Molnar <mingo@kernel.org>
> CC: Borislav Petkov <bp@alien8.de>
> CC: "H. Peter Anvin" <hpa@zytor.com>
> CC: Andy Lutomirski <luto@amacapital.net>
> CC: Oleg Nesterov <oleg@redhat.com>
> CC: Frederic Weisbecker <fweisbec@gmail.com>
> CC: Alexei Starovoitov <ast@plumgrid.com>
> CC: Will Drewry <wad@chromium.org>
> CC: Kees Cook <keescook@chromium.org>
> CC: x86@kernel.org
> CC: linux-kernel@vger.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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