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

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Message-ID: <20170518071737.xdpiusewqsuv2v55@gmail.com>
Date:   Thu, 18 May 2017 09:17:37 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] X86: don't report PAT on CPUs that don't support it


* Mikulas Patocka <mpatocka@...hat.com> wrote:

> On Tue, 16 May 2017, H. Peter Anvin wrote:
> 
> > 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
> 
> I use it for playing music, browsing with the links browser and connecting 
> to other machines with ssh. That machine is slow but it is completely 
> quiet.
> 
> It is also good to run my own software on a slow CPU to make sure that 
> there are no obvious inefficiencies.
> 
> > if so, are you actually dependent on write combining on it?  The reason
> 
> Those K6-3 MTRRs improve framebuffer write throughput by 33%.
> 
> > 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.
> 
> MTRRs are also needed on Pentium 3, Core Solo and Core Duo due to an 
> erratum that makes it not possible to set WC with PAT. See the comment 
> before "clear_cpu_cap(c, X86_FEATURE_PAT)" in early_init_intel().

Ok, I'm inclined to apply your regression fix - hpa do you concur?

Thanks,

	Ingo