linux-kernel - Re: runtime regression with "x86/mm/pat: Emulate PAT when it is disabled"

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Message-ID: <20160311134000.GC4347@pd.tnic>
Date:	Fri, 11 Mar 2016 14:40:00 +0100
From:	Borislav Petkov <bp@...e.de>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Toshi Kani <toshi.kani@....com>,
	Richard Purdie <richard.purdie@...uxfoundation.org>,
	Toshi Kani <toshi.kani@...com>,
	Bruce Ashfield <bruce.ashfield@...driver.com>,
	"Hart, Darren" <darren.hart@...el.com>,
	"saul.wold" <saul.wold@...el.com>, linux-kernel@...r.kernel.org,
	kvm ML <kvm@...r.kernel.org>, x86-ml <x86@...nel.org>
Subject: Re: runtime regression with "x86/mm/pat: Emulate PAT when it is
 disabled"

On Fri, Mar 11, 2016 at 01:23:56PM +0000, One Thousand Gnomes wrote:
> Pentium Pro has MTRR, PAT came later.

Yep, this page says so too:

http://www.cpu-world.com/CPUs/Pentium-II/Intel-Pentium%20Pro%20200%201%20MB%20-%20GJ80521EX200%201M%20%28BP80521200%201M%29.html

> I believe the qemu32 CPU isn't a "real" CPU type therefore.

And why is that so? I suspect it has again something to do with
migration and fun. Let me add the kvm ML.

Guys, does anyone have an idea why

#define PPRO_FEATURES (CPUID_FP87 | CPUID_DE | CPUID_PSE | CPUID_TSC | \
          CPUID_MSR | CPUID_MCE | CPUID_CX8 | CPUID_PGE | CPUID_CMOV | \
          CPUID_PAT | CPUID_FXSR | CPUID_MMX | CPUID_SSE | CPUID_SSE2 | \
          CPUID_PAE | CPUID_SEP | CPUID_APIC)

has CPUID_PAT *instead* of CPUID_MTRR?

If that define is really supposed to be a bitfield of Pentium Pro
features, then it should be:

#define PPRO_FEATURES (CPUID_FP87 | CPUID_DE | CPUID_PSE | CPUID_TSC | \
          CPUID_MSR | CPUID_MCE | CPUID_CX8 | CPUID_PGE | CPUID_CMOV | \
          CPUID_MTRR | CPUID_FXSR | CPUID_MMX | CPUID_SSE | CPUID_SSE2 | \
          CPUID_PAE | CPUID_SEP | CPUID_APIC)

Hmmm?

Btw,

#define PENTIUM2_FEATURES (PENTIUM_FEATURES | CPUID_PAE | CPUID_SEP | \
          CPUID_MTRR | CPUID_PGE | CPUID_MCA | CPUID_CMOV | CPUID_PAT | \
          CPUID_PSE36 | CPUID_FXSR)

looks correct in that it has MTRR but it has PAT too.

And that looks wrong to me too since, according to Wikipedia, Pentium
III was the first to sport PAT:

https://en.wikipedia.org/wiki/Page_attribute_table

More hmmmm...

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--