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Re: [Xen-devel] Any available page table entry bits in Xen??

  • To: Xen-devel@xxxxxxxxxxxxxxxxxxx
  • From: "Mike Sun" <msun@xxxxxxxxxx>
  • Date: Sat, 18 Oct 2008 22:14:25 -0400
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Hi Min,

> I'd like to know if there's any available bit in page table entries, on
> x86_32 (no pae),
> especially on L1 page table entry. It seems to me that all bits are used by
> Xen.

Could you clarify?  Are you referring to PTEs for para-virtualized
guests (without translation) or fully translated domains like HVM
guest with shadow paging?

I think for PV guests you're right, the PV guest OS cannot use of the
bits in the PTE.  Some of those bits are used by Xen (though somehow I
vaguely remember that though Xen reserves the use of those bits, they
may not actually be used).

For HVM guests, the guest PTEs will carry whatever the guest OS uses,
and in the shadow PTEs, I believe there are some bits not used by Xen
(not totally sure).


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