xen: make xen_load_gdt simpler

Remove use of multicall machinery which is unused (gdt loading
is never performance critical).  This removes the implicit use
of percpu variables, which simplifies understanding how
the percpu code's use of load_gdt interacts with this code.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/xen/enlighten.c

@@ -284,12 +284,11 @@ static void xen_set_ldt(const void *addr, unsigned entries)
 
 static void xen_load_gdt(const struct desc_ptr *dtr)
 {
-	unsigned long *frames;
 	unsigned long va = dtr->address;
 	unsigned int size = dtr->size + 1;
 	unsigned pages = (size + PAGE_SIZE - 1) / PAGE_SIZE;
+	unsigned long frames[pages];
 	int f;
-	struct multicall_space mcs;
 
 	/* A GDT can be up to 64k in size, which corresponds to 8192
 	   8-byte entries, or 16 4k pages.. */
@@ -297,9 +296,6 @@ static void xen_load_gdt(const struct desc_ptr *dtr)
 	BUG_ON(size > 65536);
 	BUG_ON(va & ~PAGE_MASK);
 
-	mcs = xen_mc_entry(sizeof(*frames) * pages);
-	frames = mcs.args;
-
 	for (f = 0; va < dtr->address + size; va += PAGE_SIZE, f++) {
 		int level;
 		pte_t *ptep = lookup_address(va, &level);
@@ -314,13 +310,15 @@ static void xen_load_gdt(const struct desc_ptr *dtr)
 
 		frames[f] = mfn;
 
+		printk("xen_load_gdt: %d va=%p mfn=%lx pfn=%lx va'=%p\n",
+		       f, (void *)va, mfn, pfn, virt);
+
 		make_lowmem_page_readonly((void *)va);
 		make_lowmem_page_readonly(virt);
 	}
 
-	MULTI_set_gdt(mcs.mc, frames, size / sizeof(struct desc_struct));
-
-	xen_mc_issue(PARAVIRT_LAZY_CPU);
+	if (HYPERVISOR_set_gdt(frames, size / sizeof(struct desc_struct)))
+		BUG();
 }
 
 static void load_TLS_descriptor(struct thread_struct *t,