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

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Date:	Thu, 17 Mar 2016 19:56:47 +0100
From:	"Luis R. Rodriguez" <mcgrof@...nel.org>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	"Luis R. Rodriguez" <mcgrof@...nel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Keir Fraser <keir@....org>, Juergen Gross <jgross@...e.com>,
	X86 ML <x86@...nel.org>, Toshi Kani <toshi.kani@...com>,
	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>
Subject: Re: [Xen-devel] MTRR on Xen - BIOS use and implications for Linux

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, in
> > particular UC settings, this concern is raised above. Another issue
> > though is that the kernel would be "unable to verify if a large page
> > mapping is aligned with MTRRs" [3]
> 
> This is not a relevant concern for Xen guests:   PV guests do not
> support superpage mappings 

And only with superpage mappings would such things be an issue? Can you
clarify why? Is there no plans to support this in the future? If the
MTRRs will not be used, to be safe, why not just skip all of them from
the e820 map and be done with it?

> and HVM guests never see real MTRRs.

Thanks, can you clarify why?

  Luis