From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753015Ab1GFJj6 (ORCPT ); Wed, 6 Jul 2011 05:39:58 -0400 Received: from mail-vx0-f174.google.com ([209.85.220.174]:62425 "EHLO mail-vx0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751914Ab1GFJj5 convert rfc822-to-8bit (ORCPT ); Wed, 6 Jul 2011 05:39:57 -0400 MIME-Version: 1.0 In-Reply-To: <1309870814-14181-1-git-send-email-ian.campbell@citrix.com> References: <20110705111525.GR17941@sun> <1309870814-14181-1-git-send-email-ian.campbell@citrix.com> Date: Wed, 6 Jul 2011 12:39:56 +0300 X-Google-Sender-Auth: Rr0fsspgxiukUB5n_Ao8APlIjXs Message-ID: Subject: Re: [PATCH 1/3] x86: drop unnecessary kernel_eflags variable from 64 bit. From: Pekka Enberg To: Ian Campbell Cc: Cyrill Gorcunov , linux-kernel@vger.kernel.org, Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , Brian Gerst , Jan Beulich , Peter Zijlstra , Frederic Weisbecker , x86@kernel.org, Andi Kleen , Linus Torvalds Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 5, 2011 at 4:00 PM, Ian Campbell wrote: > For no reason that I can determine 64 bit x86 saves the current eflags > in cpu_init purely for use in ret_from_fork. The equivalent 32 bit > code simply hard codes 0x0202 as the new EFLAGS which seems safer than > relying on a potentially arbitrary EFLAGS saved during cpu_init. > > Original i386 changeset > http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=47a5c6fa0e204a2b63309c648bb2fde36836c826 > Original x86_64 changset > http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=658fdbef66e5e9be79b457edc2cbbb3add840aa9 > > The only comment in the later indicates that it is following the > former, but not why it differs in this way. Well lets ask Linus and Andi? > This change makes 64 bit use the same mechanism to setup the initial > EFLAGS on fork. Note that 64 bit resets EFLAGS before calling > schedule_tail() as opposed to 32 bit which calls schedule_tail() > first. Therefore the correct value for EFLAGS has opposite IF > bit. This will be fixed in a subsequent patch. > > Signed-off-by: Ian Campbell > Cc: x86@kernel.org > --- >  arch/x86/kernel/cpu/common.c |    4 ---- >  arch/x86/kernel/entry_64.S   |    2 +- >  2 files changed, 1 insertions(+), 5 deletions(-) > > diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c > index 22a073d..178c3b4 100644 > --- a/arch/x86/kernel/cpu/common.c > +++ b/arch/x86/kernel/cpu/common.c > @@ -1077,8 +1077,6 @@ void syscall_init(void) >               X86_EFLAGS_TF|X86_EFLAGS_DF|X86_EFLAGS_IF|X86_EFLAGS_IOPL); >  } > > -unsigned long kernel_eflags; > - >  /* >  * Copies of the original ist values from the tss are only accessed during >  * debugging, no special alignment required. > @@ -1231,8 +1229,6 @@ void __cpuinit cpu_init(void) >        fpu_init(); >        xsave_init(); > > -       raw_local_save_flags(kernel_eflags); > - >        if (is_uv_system()) >                uv_cpu_init(); >  } > diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S > index e13329d..b50d142 100644 > --- a/arch/x86/kernel/entry_64.S > +++ b/arch/x86/kernel/entry_64.S > @@ -396,7 +396,7 @@ ENTRY(ret_from_fork) > >        LOCK ; btr $TIF_FORK,TI_flags(%r8) > > -       pushq_cfi kernel_eflags(%rip) > +       pushq_cfi $0x0002 >        popfq_cfi                               # reset kernel eflags > >        call schedule_tail                      # rdi: 'prev' task parameter