static void xen_write_gdt_entry(struct desc_struct *dt, int entry,
				const void *desc, int type)
{
	trace_xen_cpu_write_gdt_entry(dt, entry, desc, type);

	preempt_disable();

	switch (type) {
	case DESC_LDT:
	case DESC_TSS:
		
		break;

	default: {
		xmaddr_t maddr = arbitrary_virt_to_machine(&dt[entry]);

		xen_mc_flush();
		if (HYPERVISOR_update_descriptor(maddr.maddr, *(u64 *)desc))
			BUG();
	}

	}

	preempt_enable();
}

static void xen_write_ldt_entry(struct desc_struct *dt, int entrynum,
				const void *ptr)
{
	xmaddr_t mach_lp = arbitrary_virt_to_machine(&dt[entrynum]);
	u64 entry = *(u64 *)ptr;

	preempt_disable();

	xen_mc_flush();
	if (HYPERVISOR_update_descriptor(mach_lp.maddr, entry))
		BUG();

	preempt_enable();
}

static void xen_load_user_cs_desc(int cpu, struct mm_struct *mm)
{
	void *gdt;
	xmaddr_t mgdt;
	u64 descriptor;
	struct desc_struct user_cs;

	gdt = &get_cpu_gdt_table(cpu)[GDT_ENTRY_DEFAULT_USER_CS];
	mgdt = virt_to_machine(gdt);

	user_cs = mm->context.user_cs;
	descriptor = (u64) user_cs.a | ((u64) user_cs.b) << 32;

	HYPERVISOR_update_descriptor(mgdt.maddr, descriptor);
}

static void
update_descriptor(union descriptor *table, union descriptor *entry)
{
#ifndef XEN
    *table = *entry;
#else
    paddr_t pa;
    pt_entry_t *ptp;

    ptp = kvtopte((vaddr_t)table);
    pa = (*ptp & PG_FRAME) | ((vaddr_t)table & ~PG_FRAME);
    if (HYPERVISOR_update_descriptor(pa, entry->raw[0], entry->raw[1]))
        panic("HYPERVISOR_update_descriptor failed\n");
#endif
}

/*
 * Version of write_gdt_entry for use at early boot-time needed to
 * update an entry as simply as possible.
 */
static void __init xen_write_gdt_entry_boot(struct desc_struct *dt, int entry,
					    const void *desc, int type)
{
	switch (type) {
	case DESC_LDT:
	case DESC_TSS:
		/* ignore */
		break;

	default: {
		xmaddr_t maddr = virt_to_machine(&dt[entry]);

		if (HYPERVISOR_update_descriptor(maddr.maddr, *(u64 *)desc))
			dt[entry] = *(struct desc_struct *)desc;
	}

	}
}