From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752703AbaJTSwl (ORCPT ); Mon, 20 Oct 2014 14:52:41 -0400 Received: from mail-la0-f50.google.com ([209.85.215.50]:34409 "EHLO mail-la0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751233AbaJTSwj (ORCPT ); Mon, 20 Oct 2014 14:52:39 -0400 MIME-Version: 1.0 In-Reply-To: References: From: Andy Lutomirski Date: Mon, 20 Oct 2014 11:52:16 -0700 Message-ID: Subject: Re: [PATCH v3 2/2] x86_64: Don't save flags on context switch To: Thomas Gleixner , X86 ML , Ingo Molnar , "H. Peter Anvin" Cc: Sebastian Lackner , Anish Bhatt , "linux-kernel@vger.kernel.org" , Chuck Ebbert , Andy Lutomirski Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 1, 2014 at 11:28 AM, Andy Lutomirski wrote: > Now that the kernel always runs with clean flags (in particular, NT > is clear), there is no need to save and restore flags on every > context switch. Since I'm liable to forget about this, and it's a nice speedup, I figured I'd remind you all, too :) (Really crude benchmarking in KVM: context switches take around 750ns, and it's maybe 10ns faster with this patch. Nothing earth-shattering, but it's still nice.) --Andy > > Signed-off-by: Andy Lutomirski > --- > arch/x86/include/asm/switch_to.h | 12 ++++++++---- > 1 file changed, 8 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h > index d7f3b3b78ac3..751bf4b7bf11 100644 > --- a/arch/x86/include/asm/switch_to.h > +++ b/arch/x86/include/asm/switch_to.h > @@ -79,12 +79,12 @@ do { \ > #else /* CONFIG_X86_32 */ > > /* frame pointer must be last for get_wchan */ > -#define SAVE_CONTEXT "pushf ; pushq %%rbp ; movq %%rsi,%%rbp\n\t" > -#define RESTORE_CONTEXT "movq %%rbp,%%rsi ; popq %%rbp ; popf\t" > +#define SAVE_CONTEXT "pushq %%rbp ; movq %%rsi,%%rbp\n\t" > +#define RESTORE_CONTEXT "movq %%rbp,%%rsi ; popq %%rbp\t" > > #define __EXTRA_CLOBBER \ > , "rcx", "rbx", "rdx", "r8", "r9", "r10", "r11", \ > - "r12", "r13", "r14", "r15" > + "r12", "r13", "r14", "r15", "flags" > > #ifdef CONFIG_CC_STACKPROTECTOR > #define __switch_canary \ > @@ -100,7 +100,11 @@ do { \ > #define __switch_canary_iparam > #endif /* CC_STACKPROTECTOR */ > > -/* Save restore flags to clear handle leaking NT */ > +/* > + * There is no need to save or restore flags, because flags are always > + * clean in kernel mode, with the possible exception of IOPL. Kernel IOPL > + * has no effect. > + */ > #define switch_to(prev, next, last) \ > asm volatile(SAVE_CONTEXT \ > "movq %%rsp,%P[threadrsp](%[prev])\n\t" /* save RSP */ \ > -- > 1.9.3 > -- Andy Lutomirski AMA Capital Management, LLC