From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner+w=401wt.eu-S1763751AbYEHO7d@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1763751AbYEHO7d (ORCPT <rfc822;w@1wt.eu>);
	Thu, 8 May 2008 10:59:33 -0400
Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755143AbYEHO7J
	(ORCPT <rfc822;linux-kernel-outgoing>);
	Thu, 8 May 2008 10:59:09 -0400
Received: from www.tglx.de ([62.245.132.106]:43347 "EHLO www.tglx.de"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1760402AbYEHO7F (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Thu, 8 May 2008 10:59:05 -0400
Date: Thu, 8 May 2008 16:53:13 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Rene Herman <rene.herman@keyaccess.nl>
cc: "H. Peter Anvin" <hpa@zytor.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Adrian Bunk <bunk@kernel.org>, Yinghai Lu <yhlu.kernel@gmail.com>,
       Ingo Molnar <mingo@elte.hu>,
       Linux Kernel <linux-kernel@vger.kernel.org>, akpm@linux-foundation.org,
       Pavel Machek <pavel@suse.cz>
Subject: Re: [PATCH] x86: introduce a new Linux defined feature flag for PAT
 support
In-Reply-To: <482311C9.2010603@keyaccess.nl>
Message-ID: <alpine.LFD.1.10.0805081650440.3318@apollo.tec.linutronix.de>
References: <48210A71.1060409@keyaccess.nl> <86802c440805061939q39ff5500h3c9e229ecbc6b2e6@mail.gmail.com> <20080507124650.GD29935@cs181133002.pp.htv.fi> <alpine.LFD.1.10.0805072122270.3318@apollo.tec.linutronix.de> <48221AE3.6020602@keyaccess.nl>
 <482233F0.7040000@zytor.com> <48224318.8020209@keyaccess.nl> <48224361.5080102@zytor.com> <48224507.8010102@keyaccess.nl> <alpine.LFD.1.10.0805071716060.3024@woody.linux-foundation.org> <48224930.9030901@keyaccess.nl> <48225DEC.2030502@keyaccess.nl>
 <alpine.LFD.1.10.0805071857430.3024@woody.linux-foundation.org> <4822614B.6080706@zytor.com> <alpine.LFD.1.10.0805071923030.3024@woody.linux-foundation.org> <48226569.7010906@zytor.com> <alpine.LFD.1.10.0805081307520.3318@apollo.tec.linutronix.de>
 <482311C9.2010603@keyaccess.nl>
User-Agent: Alpine 1.10 (LFD 962 2008-03-14)
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, 8 May 2008, Rene Herman wrote:
> However, I'm not sure, but:
> 
> > +	/* Paranoia check. */
> > +	if (!cpu_has_pat) {
> > +		printk(KERN_ERR "PAT enabled, but CPU feature cleared\n");
> > +		/*
> > +		 * Panic if this happens on the secondary CPU, and we
> > +		 * switched to PAT on the boot CPU. We have no way to
> > +		 * undo PAT.
> > +		*/
> > +		BUG_ON(boot_pat_state);
> > +	}
> 
> The 'if this happens on the secondary CPU' sounds a bit like this is
> directly checking the secondary CPU flag but cpu_has_pat translates into
> boot_cpu_has(X86_FEATURE_PAT), refers always to the boot cpu.

Right and thats fine because of:

        /*
         * On SMP, boot_cpu_data holds the common feature set between
         * all CPUs; so make sure that we indicate which features are
         * common between the CPUs.  The first time this routine gets
         * executed, c == &boot_cpu_data.
         */
        if (c != &boot_cpu_data) {
                /* AND the already accumulated flags with these */
                for (i = 0 ; i < NCAPINTS ; i++)
                        boot_cpu_data.x86_capability[i] &= c->x86_capability[i];
        }

Thanks,
	tglx