Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757323AbbDVQyr (ORCPT ); Wed, 22 Apr 2015 12:54:47 -0400 Received: from mail-lb0-f169.google.com ([209.85.217.169]:35952 "EHLO mail-lb0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754351AbbDVQyq (ORCPT ); Wed, 22 Apr 2015 12:54:46 -0400 MIME-Version: 1.0 In-Reply-To: <1429720808-7173-2-git-send-email-dvlasenk@redhat.com> References: <1429720808-7173-1-git-send-email-dvlasenk@redhat.com> <1429720808-7173-2-git-send-email-dvlasenk@redhat.com> From: Andy Lutomirski Date: Wed, 22 Apr 2015 09:54:24 -0700 Message-ID: Subject: Re: [PATCH 2/2] x86/asm/entry/32: Remove unnecessary optimization in stub32_clone To: Denys Vlasenko , Josh Triplett Cc: Ingo Molnar , Linus Torvalds , Steven Rostedt , Borislav Petkov , "H. Peter Anvin" , Oleg Nesterov , Frederic Weisbecker , Alexei Starovoitov , Will Drewry , Kees Cook , X86 ML , "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2252 Lines: 64 On Wed, Apr 22, 2015 at 9:40 AM, Denys Vlasenko 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 > CC: Linus Torvalds > CC: Steven Rostedt > CC: Ingo Molnar > CC: Borislav Petkov > CC: "H. Peter Anvin" > CC: Andy Lutomirski > CC: Oleg Nesterov > CC: Frederic Weisbecker > CC: Alexei Starovoitov > CC: Will Drewry > CC: Kees Cook > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/