xen: Add PVH boot entry code

Add the code for the Xen PVH mode boot entry.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Tested-by: Hans van Kranenburg <hans@knorrie.org>

grub-core/kern/i386/xen/startup_pvh.S

@@ -19,11 +19,63 @@
 
 #include <config.h>
 #include <grub/symbol.h>
+#include <grub/machine/memory.h>
 
 	.file	"startup_pvh.S"
 	.text
+	.globl	start, _start
+	.code32
 
+start:
+_start:
+	cld
+	lgdt	gdtdesc
+	ljmp	$GRUB_MEMORY_MACHINE_PROT_MODE_CSEG, $1f
+1:
+	movl	$GRUB_MEMORY_MACHINE_PROT_MODE_DSEG, %eax
+	mov	%eax, %ds
+	mov	%eax, %es
+	mov	%eax, %fs
+	mov	%eax, %gs
+	mov	%eax, %ss
+	leal	LOCAL(stack_end), %esp
+
+	/* Save address of start info structure. */
+	mov	%ebx, pvh_start_info
+	call	EXT_C(grub_main)
+	/* Doesn't return. */
+
+	.p2align	3
+gdt:
+	.word	0, 0
+	.byte	0, 0, 0, 0
+
+	/* -- code segment --
+	 * base = 0x00000000, limit = 0xFFFFF (4 KiB Granularity), present
+	 * type = 32bit code execute/read, DPL = 0
+	 */
+	.word	0xFFFF, 0
+	.byte	0, 0x9A, 0xCF, 0
+
+	/* -- data segment --
+	 * base = 0x00000000, limit 0xFFFFF (4 KiB Granularity), present
+	 * type = 32 bit data read/write, DPL = 0
+	 */
+	.word	0xFFFF, 0
+	.byte	0, 0x92, 0xCF, 0
+
+	.p2align	3
+/* this is the GDT descriptor */
+gdtdesc:
+	.word	0x17		/* limit */
+	.long	gdt		/* addr */
+
+	.p2align	2
 /* Saved pointer to start info structure. */
 	.globl	pvh_start_info
 pvh_start_info:
 	.long	0
+
+	.bss
+	.space	GRUB_MEMORY_MACHINE_PROT_STACK_SIZE
+LOCAL(stack_end):