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

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Date:	Thu, 08 May 2008 09:48:11 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Rene Herman <rene.herman@...access.nl>,
	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

Thomas Gleixner wrote:
>> 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.

No, it doesn't.  Although it is badly named, it refers to the common 
capability set of all online CPUs.

> 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];
>         }

... because of this code. :)

	-hpa
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