From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761096Ab3BJEgA (ORCPT ); Sat, 9 Feb 2013 23:36:00 -0500 Received: from terminus.zytor.com ([198.137.202.10]:47329 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1761080Ab3BJEf7 (ORCPT ); Sat, 9 Feb 2013 23:35:59 -0500 Message-ID: <5117236D.5000607@zytor.com> Date: Sat, 09 Feb 2013 20:34:53 -0800 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130110 Thunderbird/17.0.2 MIME-Version: 1.0 To: Borislav Petkov CC: X86 ML , LKML , Borislav Petkov Subject: Re: [PATCH 2/5 -v2.1] x86: Detect CPUID support early at boot References: <20130209231351.GB5556@pd.tnic> <1360451762-10175-1-git-send-email-bp@alien8.de> In-Reply-To: <1360451762-10175-1-git-send-email-bp@alien8.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/09/2013 03:16 PM, Borislav Petkov wrote: > From: Borislav Petkov > > We detect CPUID function support on each CPU and save it for later use, > obviating the need to play the toggle EFLAGS.ID game every time. C code > is looking at ->cpuid_level anyway. > > Signed-off-by: Borislav Petkov > --- > arch/x86/kernel/head_32.S | 48 +++++++++++++++++++++++------------------------ > 1 file changed, 23 insertions(+), 25 deletions(-) > > diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S > index f4d919e2cd2b..534397ba226c 100644 > --- a/arch/x86/kernel/head_32.S > +++ b/arch/x86/kernel/head_32.S > @@ -318,30 +318,37 @@ default_entry: > movl %eax,%cr0 > > /* > - * New page tables may be in 4Mbyte page mode and may > - * be using the global pages. > + * Initialize EFLAGS. Some BIOSes leave bits like NT set. This would confuse the > + * debugger if this code is traced. Best to initialize before switching to > + * protected mode. As a side effect, we clear DF too because GCC expects it so. > + */ > + pushl $0 > + popfl > + I wouldn't really call it a "side effect". Perhaps the right thing here is to say something like "we want to start out with %eflags unambiguously clear". (Note also we have had to CLD earlier because we have already copied the command line.) -hpa -- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.