Re: mapping of memory below 16Mb

["Jan Beulich" <jbeulich@novell.com>]

>>> Keir Fraser <Keir.Fraser@cl.cam.ac.uk> 24.01.08 17:27 >>>
>On 24/1/08 16:24, "Jan Beulich" <jbeulich@novell.com> 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.
>> 
>> Oh, and my understanding of PAT support goes beyond just setting the
>> PAT MSR and adjusting page table flags. Specifically, if we have PAT we
>> should never need to modify MTRRs, which would eliminate conflicts on
>> updates of those that I'm seeing on various boxes, always just
>> depending on how BIOS sets them up, and always related to overlaps
>> of the video memory with existing MTRR ranges.
>
>That's a dom0 issue. Xen itself does not proactively mess with the MTRRs.

Yes and no: Yes in the sense that Dom0 could be given an alternative
hypercall to specify memory types. No in the sense that we already have
such a hypercall (setting the MTRRs) that would simply need to do
translation to PAT semantics rather than writing to the MTRR MSRs.

Jan