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

From "Luis R. Rodriguez" <mcgrof@kernel.org>
Newsgroups linux.kernel
Subject Re: [Xen-devel] MTRR on Xen - BIOS use and implications for Linux
Date 2016-03-17 20:00 +0100
Message-ID <rdLjk-2CT-11@gated-at.bofh.it> (permalink)
References <rdpVw-5jX-21@gated-at.bofh.it> <rdE89-6IF-1@gated-at.bofh.it>
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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

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Thread

MTRR on Xen - BIOS use and implications for Linux "Luis R. Rodriguez" <mcgrof@kernel.org> - 2016-03-16 21:10 +0100
  Re: [Xen-devel] MTRR on Xen - BIOS use and implications for Linux David Vrabel <david.vrabel@citrix.com> - 2016-03-17 12:20 +0100
    Re: [Xen-devel] MTRR on Xen - BIOS use and implications for Linux "Luis R. Rodriguez" <mcgrof@kernel.org> - 2016-03-17 20:00 +0100
      Re: [Xen-devel] MTRR on Xen - BIOS use and implications for Linux "Luis R. Rodriguez" <mcgrof@kernel.org> - 2016-03-29 19:30 +0200
        Re: [Xen-devel] MTRR on Xen - BIOS use and implications for Linux Toshi Kani <toshi.kani@hpe.com> - 2016-03-29 23:30 +0200
          Re: [Xen-devel] MTRR on Xen - BIOS use and implications for Linux "Luis R. Rodriguez" <mcgrof@kernel.org> - 2016-03-30 00:30 +0200

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