[PATCH 18/18] x86: Use a simple jump into long mode

[Simon Glass]

With the 64-bit descriptor we can use a jump instruction, rather than
pushing things on the stack.

Since the processor is in 64-bit mode by this point, pop a 64-bit value
from the stack, containing the target address.

This simplifies the code slightly, in particular its use of the stack.

Signed-off-by: Simon Glass <sjg at chromium.org>
---

 arch/x86/cpu/i386/call64.S | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/arch/x86/cpu/i386/call64.S b/arch/x86/cpu/i386/call64.S
index 3137ec17d31..c6308b92e25 100644
--- a/arch/x86/cpu/i386/call64.S
+++ b/arch/x86/cpu/i386/call64.S
@@ -22,6 +22,7 @@ cpu_call64:
 	 * ecx - target
 	 */
 	cli
+	pushl	$0		/* top 64-bits of target */
 	push	%ecx		/* arg2 = target */
 	push	%edx		/* arg1 = setup_base */
 	mov	%eax, %ebx
@@ -32,7 +33,8 @@ cpu_call64:
 	movl	%eax, %cr0
 
 	/* Enable PAE mode */
-	movl	$(X86_CR4_PAE), %eax
+	movl	%cr4, %eax
+	orl	$X86_CR4_PAE, %eax
 	movl	%eax, %cr4
 
 	/* Enable the boot page tables */
@@ -57,23 +59,18 @@ cpu_call64:
 	 */
 	pop	%esi			/* setup_base */
 
-	pushl	$(X86_GDT_ENTRY_64BIT_CS * X86_GDT_ENTRY_SIZE)
-	leal	lret_target, %eax
-	pushl	%eax
-
 	/* Enter paged protected Mode, activating Long Mode */
 	movl	%cr0, %eax
 	orl	$X86_CR0_PG, %eax
 	movl	%eax, %cr0
 
 	/* Jump from 32bit compatibility mode into 64bit mode. */
-	lret
+	ljmp	$(X86_GDT_ENTRY_64BIT_CS * X86_GDT_ENTRY_SIZE), $lret_target
 
-code64:
+.code64
 lret_target:
-	pop	%eax			/* target */
-	mov	%eax, %eax		/* Clear bits 63:32 */
-	jmp	*%eax			/* Jump to the 64-bit target */
+	pop	%rax			/* target */
+	jmp	*%rax			/* Jump to the 64-bit target */
 
 .globl call64_stub_size
 call64_stub_size:
-- 
2.34.1