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Date: Thu, 10 Mar 2016 14:04:29 -0500
From: Paul Gortmaker <paul.gortmaker@windriver.com>
To: Borislav Petkov <bp@suse.de>
CC: Toshi Kani <toshi.kani@hpe.com>,
        Richard Purdie <richard.purdie@linuxfoundation.org>,
        Toshi Kani <toshi.kani@hp.com>,
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        "Hart, Darren" <darren.hart@intel.com>,
        "saul.wold" <saul.wold@intel.com>, <linux-kernel@vger.kernel.org>
Subject: Re: runtime regression with "x86/mm/pat: Emulate PAT when it is
 disabled"
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[Re: runtime regression with "x86/mm/pat: Emulate PAT when it is disabled"] On 10/03/2016 (Thu 18:20) Borislav Petkov wrote:

> 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.

So, I guess that is a qemu bug?  If there is no real silicon out there
that has no MTRR but does claim PAT, then qemu32 is a flawed CPU type?

> 
> 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.

That is probably more of an RP question.  In principle I guess other CPU
types are on the table, and most likely qemu32 is just there from
historical reasons.  We do know that we don't want "-cpu host" though,
since that will introduce variability into the automated testing.

Paul.
--

> 
> :-)
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
> --