x86: Use the same GDT when jumping to long mode

Make use the existing GDT which now includes entries for 64-bit code.
Leave the interrupt descriptors alone. They can be tidied up once U-Boot
starts up.

With this, kvm mode works with QEMU.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: https://source.denx.de/u-boot/custodians/u-boot-dm/-/issues/31

arch/x86/cpu/i386/call64.S

@@ -7,6 +7,7 @@
  */
 
 #include <asm/msr-index.h>
+#include <asm/processor.h>
 #include <asm/processor-flags.h>
 
 .code32
@@ -30,11 +31,6 @@ cpu_call64:
 	andl	$~X86_CR0_PG, %eax
 	movl	%eax, %cr0
 
-	/* Load new GDT with the 64bit segments using 32bit descriptor */
-	leal	gdt, %eax
-	movl	%eax, gdt+2
-	lgdt	gdt
-
 	/* Enable PAE mode */
 	movl	$(X86_CR4_PAE), %eax
 	movl	%eax, %cr4
@@ -49,12 +45,6 @@ cpu_call64:
 	btsl	$_EFER_LME, %eax
 	wrmsr
 
-	/* After gdt is loaded */
-	xorl	%eax, %eax
-	lldt	%ax
-	movl    $0x20, %eax
-	ltr	%ax
-
 	/*
 	 * Setup for the jump to 64bit mode
 	 *
@@ -67,7 +57,7 @@ cpu_call64:
 	 */
 	pop	%esi			/* setup_base */
 
-	pushl	$0x10
+	pushl	$(X86_GDT_ENTRY_64BIT_CS * X86_GDT_ENTRY_SIZE)
 	leal	lret_target, %eax
 	pushl	%eax