Re: [Xen-devel] Re: Xen-devel Digest, Vol 25, Issue 93

[Keir Fraser <keir@xxxxxxxxxxxxx>]

On 12/3/07 16:19, "Petersson, Mats" <Mats.Petersson@xxxxxxx> wrote:

>> I have seen in the code that there are different cases of guest+host
>> paging table heights. Why?
> 
> I'm sorry, I don't quite know this. I believe that the page-table has to
> be the same number of levels in both Xen and the PV guest.
> 
> There's been some recent work to implement 32-bit PV on 64-bit HV, which
> I think changes this by allowing a 32-bit PAE guest to run on a 64-bit
> hypervisor. Someone else who works more on PV is probably better to
> answer this... 

For PV guests, there are no separate Xen/shadow page tables. Xen reserves a
bit of space at the top end of guest pagetables to map itself. Hence
normally the guest and Xen pagetables must be the same height as they are
actually the same pagetables.

Supporting PAE guest on 64-bit Xen is the only exception. Xen maintains a
hidden top-level page directory and one of the entries in that directory
points at the guest's three-level pagetable. But again there is no shadowing
of the guest three-level pagetable: they are directly hooked into the hidden
top-level directory, and the real physical %cr3 points at that hidden
directory.

 -- Keir


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