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

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Message-ID: <56EA913F.1040403@citrix.com>
Date:	Thu, 17 Mar 2016 11:13:03 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	"Luis R. Rodriguez" <mcgrof@...nel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Keir Fraser <keir@....org>
CC:	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

tl;dr?

Somewhere in here are some actual questions which I will attempt to
answer. Please try and be more concise in future.

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 HVM guests never see real MTRRs.

David