x86/vdso: Change the PER_CPU segment to use struct desc_struct

This makes it easier to see what's going on.  It produces
exactly the same segment descriptor as the old code.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/d492f7b55136cbc60f016adae79160707b2e03b7.1411494540.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/vdso/vma.c

@@ -248,7 +248,7 @@ __setup("vdso=", vdso_setup);
  */
 static void vsyscall_set_cpu(int cpu)
 {
-	unsigned long d;
+	struct desc_struct d;
 	unsigned long node = 0;
 #ifdef CONFIG_NUMA
 	node = cpu_to_node(cpu);
@@ -257,13 +257,18 @@ static void vsyscall_set_cpu(int cpu)
 		write_rdtscp_aux((node << 12) | cpu);
 
 	/*
-	 * Store cpu number in limit so that it can be loaded quickly
-	 * in user space in vgetcpu. (12 bits for the CPU and 8 bits for the node)
+	 * Store cpu number in limit so that it can be loaded
+	 * quickly in user space in vgetcpu. (12 bits for the CPU
+	 * and 8 bits for the node)
 	 */
-	d = 0x0f40000000000ULL;
-	d |= cpu;
-	d |= (node & 0xf) << 12;
-	d |= (node >> 4) << 48;
+	d = (struct desc_struct) {
+		.limit0 = cpu | ((node & 0xf) << 12),
+		.limit = node >> 4,
+		.type = 4,		/* RO data, expand down */
+		.dpl = 3,		/* Visible to user code */
+		.s = 1,			/* Not a system segment */
+		.p = 1,			/* Present */
+	};
 
 	write_gdt_entry(get_cpu_gdt_table(cpu), GDT_ENTRY_PER_CPU, &d, DESCTYPE_S);
 }