linux-kernel - Re: [PATCH] X86: don't report PAT on CPUs that don't support it

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Message-ID: <235765f2-77dd-6210-17a1-ca67eecc5983@zytor.com>
Date:   Tue, 16 May 2017 06:57:07 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Mikulas Patocka <mpatocka@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] X86: don't report PAT on CPUs that don't support it

On 04/18/17 12:07, Mikulas Patocka wrote:
> 
> However, on AMD K6-3 CPU, the processor initialization code never calls
> pat_init() and so __pat_enabled stays 1 and the function pat_enabled()
> returns true, even though the K6-3 CPU doesn't support PAT.
> 

OK, now I'm wondering: are you actually *using* said K6-3 machine, and
if so, are you actually dependent on write combining on it?  The reason
I'm asking is because I would personally like to completely remove the
support for using MTRRs to create WC mappings, as it only affects a
handful of ancient CPUs: Pentium Pro, Pentium II, K6-*, and possibly
some Cyrix/Centaur part.  Earlier CPUs didn't have WC, but could set WB,
WT or UC via the page tables without needing the PAT MSR, and newer CPUs
have PAT.

	-hpa