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Xen 3.0 Virtualization Interface Guide
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3.6 Segment Descriptor Tables

At start of day a guest is supplied with a default GDT, which does not reside within its own memory allocation. If the guest wishes to use other than the default `flat' ring-1 and ring-3 segments that this GDT provides, it must register a custom GDT and/or LDT with Xen, allocated from its own memory.

The following hypercall is used to specify a new GDT:

int set_gdt(unsigned long *frame_list, int entries)

frame_list: An array of up to 14 machine page frames within which the GDT resides. Any frame registered as a GDT frame may only be mapped read-only within the guest's address space (e.g., no writable mappings, no use as a page-table page, and so on). Only 14 pages may be specified because pages 15 and 16 are reserved for the hypervisor's GDT entries.

entries: The number of descriptor-entry slots in the GDT.

The LDT is updated via the generic MMU update mechanism (i.e., via the mmu_update() hypercall.

Xen 3.0 Virtualization Interface Guide
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  Published under the terms of the GNU General Public License Design by Interspire