[Xen-devel] Re: How to get guest PTEs info of a HVM with EPT

[KennyChen <windtracekimo@xxxxxxxxxxxx>]

Tim Deegan <Tim.Deegan <at> citrix.com> writes:

> 
> At 02:50 +0000 on 20 Jan (1295491817), KennyChen wrote:
> > When EPT is enabled for a HVM guest, is that possible for hypervisor
> > to get the PTE information of guest page table? e.g, R/W, Supervisor
> > flags? 
> 
> Yes, of course - you know the guest's CR3 value and you can map all its
> memory so you can read its pagetables.  Look at hap_gva_to_gfn() and 
> how it calls guest_walk_tables().  You can even do it from userspace 
> in dom0 - see xc_translate_foreign_address().
> 
> What you can't do easily without shadow paging is _control_ the
> pagetable values.
> 
> Cheers,
> 
> Tim.
> 

Thanks for the hint, Tim, but I still got some questions.

When I put debug dump in the guest_walk_tables() to observe the l4e entry
(64-bit guest with 4GB ram, Centos 5.5), is it normal to get a l4e "10e21a067".
It seems the gfn 0x10e21a has exceeded the maximum physical memory of the guest
(0x100000 for 4GB). Is this a bug of the guest or some tricks in the l4e?

The other weird thing is gfn_to_mfn_unshare() called inside hap_gva_to_gfn().
Why is it necessary to unshare the page when we traverse the page table? we are
just reading it right?

Kenny




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