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

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Message-ID: <20160310172029.GA2194@pd.tnic>
Date:	Thu, 10 Mar 2016 18:20:29 +0100
From:	Borislav Petkov <bp@...e.de>
To:	Toshi Kani <toshi.kani@....com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Cc:	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
Subject: Re: runtime regression with "x86/mm/pat: Emulate PAT when it is
 disabled"

On Thu, Mar 10, 2016 at 09:49:51AM -0700, Toshi Kani wrote:
> This confirms the issue - QEMU's virtual Intel CPU does not support MTRR. 
> 
> When MTRR is disabled, the kernel does not call pat_init().  pat_enabled()
> is still set to true when CONFIG_X86_PAT is set.  CONFIG_X86_PAT depends on
> CONFIG_MTRR, and assumes that MTRR is enabled. 

Aha, so "qemu32" model doesn't support MTRRs but "kvm32" does, for
example. And so do the majority of the other CPU types.

Paul, can you guys run with something else besides "qemu32"? You can
even take a 64-bit one and run a 32-bit guest on it.

:-)

-- 
Regards/Gruss,
    Boris.

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