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Xen 3.0 Virtualization Interface Guide
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3.4 Writable Page Tables

Xen also provides an alternative mode of operation in which guests have the illusion that their page tables are directly writable. Of course this is not really the case, since Xen must still validate modifications to ensure secure partitioning. To this end, Xen traps any write attempt to a memory page of type PT (i.e., that is currently part of a page table). If such an access occurs, Xen temporarily allows write access to that page while at the same time disconnecting it from the page table that is currently in use. This allows the guest to safely make updates to the page because the newly-updated entries cannot be used by the MMU until Xen revalidates and reconnects the page. Reconnection occurs automatically in a number of situations: for example, when the guest modifies a different page-table page, when the domain is preempted, or whenever the guest uses Xen's explicit page-table update interfaces.

Writable pagetable functionality is enabled when the guest requests it, using a vm_assist hypercall. Writable pagetables do not provide full virtualisation of the MMU, so the memory management code of the guest still needs to be aware that it is running on Xen. Since the guest's page tables are used directly, it must translate pseudo-physical addresses to real machine addresses when building page table entries. The guest may not attempt to map its own pagetables writably, since this would violate the memory type invariants; page tables will automatically be made writable by the hypervisor, as necessary.

Xen 3.0 Virtualization Interface Guide
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