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

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Message-ID: <482311C9.2010603@keyaccess.nl>
Date:	Thu, 08 May 2008 16:44:25 +0200
From:	Rene Herman <rene.herman@...access.nl>
To:	Thomas Gleixner <tglx@...utronix.de>
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 08-05-08 14:49, Thomas Gleixner wrote:

> Subject: x86: cleanup PAT cpu validation
> From: Thomas Gleixner <tglx@...utronix.de>
> Date: Thu, 08 May 2008 09:18:43 +0200

Thanks much. Definite ACK, very much including on the much better disable
message:

> +	pat_disable(cpu_has_pat ?
> +		    "PAT disabled. Not yet verified on this CPU type." :
> +		    "PAT not supported by CPU.");
> +}

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. Yes, we
continued on on the boot CPU after the error message so this triggers,
but thought I'd make sure it was also intended this way. If so, never
mind...

I'm privately again placing this on top. If anyone has any explicit
testing suggestion, I'm all ears.

Rene.

View attachment "pat_duron7.diff" of type "text/plain" (531 bytes)