Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757207Ab2JJT7b (ORCPT ); Wed, 10 Oct 2012 15:59:31 -0400 Received: from lunge.queued.net ([173.255.254.236]:42134 "EHLO lunge.queued.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757114Ab2JJT7a (ORCPT ); Wed, 10 Oct 2012 15:59:30 -0400 Date: Wed, 10 Oct 2012 12:59:27 -0700 From: Andres Salomon To: "H. Peter Anvin" Cc: Linux Kernel Mailing List , Ingo Molnar , Thomas Gleixner , Dave Jones , Linus Torvalds , "Eric W. Biederman" , Ian Campbell , Konrad Rzeszutek Wilk , Jeremy Fitzhardinge , Rusty Russell , David Woodhouse , Vivek Goyal , Yinghai Lu , "H. Peter Anvin" , techteam@lists.laptop.org Subject: Re: [RFC PATCH] x86-32: Start out eflags and cr4 clean Message-ID: <20121010125927.0968dd8b@dev.queued.net> In-Reply-To: <1348529239-17943-1-git-send-email-hpa@linux.intel.com> References: <87sjaaj3cx.fsf@xmission.com> <1348529239-17943-1-git-send-email-hpa@linux.intel.com> X-Mailer: Claws Mail 3.8.1 (GTK+ 2.24.10; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2866 Lines: 86 Thanks for Cc'ing me. I just tested this on an XO-1, and it doesn't appear to break anything. So, looks fine to me. Acked-by: Andres Salomon On Mon, 24 Sep 2012 16:27:19 -0700 "H. Peter Anvin" wrote: > From: "H. Peter Anvin" > > %cr4 is supposed to reflect a set of features into which the operating > system is opting in. If the BIOS or bootloader leaks bits here, this > is not desirable. Consider a bootloader passing in %cr4.pae set to a > legacy paging kernel, for example -- it will not have any immediate > effect, but the kernel would crash when turning paging on. > > A similar argument applies to %eflags, and since we have to look for > %eflags.id being settable we can use a sequence which clears %eflags > as a side effect. > > Note that we already do this for x86-64. > > I would like opinions on this especially from the PV crowd and > nonstandard platforms (e.g. OLPC) to make sure we don't screw them up. > > Signed-off-by: H. Peter Anvin > --- > arch/x86/kernel/head_32.S | 31 ++++++++++++++++--------------- > 1 files changed, 16 insertions(+), 15 deletions(-) > > diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S > index d42ab17..957a47a 100644 > --- a/arch/x86/kernel/head_32.S > +++ b/arch/x86/kernel/head_32.S > @@ -287,27 +287,28 @@ ENTRY(startup_32_smp) > leal -__PAGE_OFFSET(%ecx),%esp > > default_entry: > - > /* > * New page tables may be in 4Mbyte page mode and may > * be using the global pages. > * > * NOTE! If we are on a 486 we may have no cr4 at all! > - * So we do not try to touch it unless we really have > - * some bits in it to set. This won't work if the BSP > - * implements cr4 but this AP does not -- very unlikely > - * but be warned! The same applies to the pse feature > - * if not equally supported. --macro > - * > - * NOTE! We have to correct for the fact that we're > - * not yet offset PAGE_OFFSET.. > + * Specifically, cr4 exists if and only if CPUID exists, > + * which in turn exists if and only if EFLAGS.ID exists. > */ > -#define cr4_bits pa(mmu_cr4_features) > - movl cr4_bits,%edx > - andl %edx,%edx > - jz 6f > - movl %cr4,%eax # Turn on paging options > (PSE,PAE,..) > - orl %edx,%eax > + movl $X86_EFLAGS_ID,%ecx > + pushl %ecx > + popfl > + pushfl > + popl %eax > + pushl $0 > + popfl > + pushfl > + popl %edx > + xorl %edx,%eax > + testl %ecx,%eax > + jz 6f # No ID flag = no CPUID = no CR4 > + > + movl pa(mmu_cr4_features),%eax > movl %eax,%cr4 > > testb $X86_CR4_PAE, %al # check if PAE is > enabled -- 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/