Re: [Xen-devel] where is writable page table implemented?

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

At 14:26 -0400 on 24 Mar (1206368762), weiming wrote:
> 1) how does the guest OS tell xen which page is a pagetable page? 

With an MMUEXT_OP hypercall, if I recall correctly.

> What if the guest doesn't do so? 

Then it's not allowed to use it as a pagetable.

> I assume xen should check if a pagetable had
> already been registered or not. But in do_mmuext_op(), there is no such
> check before setting the cr3.

In new_guest_cr3(), the call to get_page_and_type_from_pagenr() will try
to take a typed reference count on the page, which marks it as a
pagetable. 

The check for paging_mode_refcounts(d) is to say that if the guest is
running under full shadow pagetables/HAP, then we skip the usual
protection mechanisms and leave it to the paging-assistance code. 

> 2) in ptwr_do_page_fault(), it should disconnect the page from page
> directory and then mark the page as writable.  Where are they done? In
> x86_emulate()?

That's not done any more.  Instead, x86_emulate() emulates the
instruction and verifies the result is OK. 

Tim.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

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