hw_x86_64: Add Genode::Gdt class

The class provides the load() function which reloads the GDTR with the
GDT address in the mtc region. This is needed to make the segments
accessible to non-core threads.

Make the _gdt_start label global to use it in the call to
_virt_mtc_addr().

repos/base-hw/src/core/include/spec/x86_64/gdt.h

@@ -0,0 +1,34 @@
+#ifndef _GDT_H_
+#define _GDT_H_
+
+#include <pseudo_descriptor.h>
+#include <mtc_util.h>
+
+extern int _gdt_start;
+
+namespace Genode
+{
+	/**
+	 * Global Descriptor Table (GDT)
+	 * See Intel SDM Vol. 3A, section 3.5.1
+	 */
+	class Gdt;
+}
+
+class Genode::Gdt
+{
+	public:
+
+		/**
+		 * Load GDT from mtc region into GDTR.
+		 *
+		 * \param virt_base  virtual base address of mode transition pages
+		 */
+		static void load(addr_t const virt_base)
+		{
+			asm volatile ("lgdt %0" : : "m" (Pseudo_descriptor (0x37,
+						  _virt_mtc_addr(virt_base, (addr_t)&_gdt_start))));
+		}
+};
+
+#endif /* _GDT_H_ */

repos/base-hw/src/core/spec/x86_64/mode_transition.s

@@ -269,6 +269,7 @@
 	.space 2
 
 	.global _gdt_ptr
+	.global _gdt_start
 	_gdt_ptr:
 	.word _gdt_end - _gdt_start - 1 /* limit        */
 	.long _gdt_start                /* base address */