linux-kernel - Re: [PATCH] x86: introduce a new Linux defined feature flag for PAT support

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Date:	Thu, 8 May 2008 16:53:13 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Rene Herman <rene.herman@...access.nl>
cc:	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Adrian Bunk <bunk@...nel.org>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	akpm@...ux-foundation.org, Pavel Machek <pavel@...e.cz>
Subject: Re: [PATCH] x86: introduce a new Linux defined feature flag for PAT
 support

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
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