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Re: [Xen-devel] mapping of memory below 16Mb

  • To: Jan Beulich <jbeulich@xxxxxxxxxx>
  • From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
  • Date: Thu, 24 Jan 2008 16:08:32 +0000
  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
  • Delivery-date: Thu, 24 Jan 2008 08:09:17 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: Acheo11qm+cS2sqWEdy57QAX8io7RQ==
  • Thread-topic: [Xen-devel] mapping of memory below 16Mb

On 24/1/08 15:57, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

>>> The WP attribute raises another question: Are there any plans to make
>>> Xen use PAT rather than (relying on) MTRRs? Linux folks seem to be
>>> finally getting closer to this, too, so even virtual PAT support may be
>>> needed in not too distant future...
>> We already support PAT for both PV guests and HVM guests.
> Hmm, I must have missed this going in. Is the indexing into PAT then
> considered part of the ABI (as I can't see either a hypercall a guest
> could use to learn what bit combination to use for certain attributes or
> one to set a virtual PAT value)?

Xen statically initialises the PAT MSR to 0x0000050100070406 and that is
part of the PV guest ABI. PAT6 and PAT7 indexes are reserved -- their memory
type may change in future.

> And regardless of that, SVM code has no sign of handling accesses to the

Oh, some VMX code needs to be common-ised then. But since this only makes
any difference for guests with direct hardware passthru, and that doesn't
work on AMD systems yet, this is hardly a priority.

 -- Keir

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