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

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Date:	Thu, 10 Mar 2016 13:12:02 -0700
From:	Toshi Kani <toshi.kani@....com>
To:	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Borislav Petkov <bp@...e.de>
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, 2016-03-10 at 14:04 -0500, Paul Gortmaker wrote:
> [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?

It turns out that your qemu's virtual CPU does not support PAT, either. :-) 
 So, it is consistent on this regard.  I will send patches to address this
issue.

Thanks,
-Toshi