linux-kernel - Re: [Xen-devel] MTRR on Xen - BIOS use and implications for Linux

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Date:	Tue, 29 Mar 2016 16:14:41 -0600
From:	Toshi Kani <toshi.kani@....com>
To:	"Luis R. Rodriguez" <mcgrof@...nel.org>,
	David Vrabel <david.vrabel@...rix.com>,
	Toshi Kani <toshi.kani@...com>
Cc:	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Keir Fraser <keir@....org>, Juergen Gross <jgross@...e.com>,
	X86 ML <x86@...nel.org>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Stuart Hayes <stuart.w.hayes@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Prarit Bhargava <prarit@...hat.com>
Subject: Re: [Xen-devel] MTRR on Xen - BIOS use and implications for Linux

On Tue, 2016-03-29 at 10:22 -0700, Luis R. Rodriguez wrote:
> On Thu, Mar 17, 2016 at 11:56 AM, Luis R. Rodriguez <mcgrof@...nel.org>
> wrote:
> > On Thu, Mar 17, 2016 at 11:13:03AM +0000, David Vrabel wrote:
> > > On 16/03/16 20:08, Luis R. Rodriguez wrote:
> > > > Toshi noted a while ago as well that if BIOS/firmware enables MTRR
> > > > but the kernel does not have it enabled one issue might have been
> > > > any MTRRs set up by the BIOS and ensuring the mapping is respected,
> 
> To be clear the requirement expressed here was needing at least to
> implement get_mtrr() on the Linux Xen guest side, it'd call the
> already implemented hypercall XENPF_read_memtype in turn. Toshi had
> hinted this was perhaps needed on the Linux Xen guest side given that
> the BIOS may have set up MTRRs on its own, so we needed the guest to
> be able to get the right type for a particular range. More on that
> below

No, I did not say that. :-)  I said it because you mentioned that you
wanted to disable MTRRs even though BIOS enabled MTRRs.

My point is that the kernel should not change the MTRR setup.  That is:
 - If BIOS enables MTRRs, then kernel needs to keep them enabled.
 - If BIOS disables MTRRs, then the kernel needs to keep them disabled.

Therefore, the kernel can keep MTRRs disabled on Xen guests.

I think you repeated the same questions in this email, so please refer my
previous reply for the rest.

Thanks,
-Toshi