apboot: avoid self-modifying code

self-modifying code is generally frowned upon, Intel largely says the
support is model-dependent.  We can as well just relocate from the C
code like we did for the temporary gdt.

2 files changed, 10 insertions, 15 deletions

diff --git a/i386/i386/cpuboot.S b/i386/i386/cpuboot.S
index b2f9e520..3e4d5420 100644
--- a/i386/i386/cpuboot.S
+++ b/i386/i386/cpuboot.S
@@ -100,7 +100,7 @@ apboot_percpu_med:
 apboot_percpu_high:
 	.byte 0
 
-.globl apboot, apbootend, gdt_descr_tmp
+.globl apboot, apbootend, gdt_descr_tmp, apboot_jmp_offset
 .align 16
 .code16
 
@@ -128,21 +128,11 @@ _apboot:
 	orl	$CR0_SET_FLAGS, %eax
 	movl	%eax, %cr0
 
-	xorl	%eax, %eax
-	mov	%cs, %ax
-	shll	$4, %eax
-	addl	$M(0f), %eax
-	movl	%eax, M(ljmp_offset32)
-
-	/* Flush cached instruction queue */
-	jmp	1f
-1:
-
-	/* ljmpl with relocation */
+	/* ljmpl with relocation from machine_init */
 	.byte 0x66
 	.byte 0xea
-ljmp_offset32:
-	.long 0xffffffff
+apboot_jmp_offset:
+	.long M(0f)
 	.word BOOT_CS
 
 0:
diff --git a/i386/i386at/model_dep.c b/i386/i386at/model_dep.c
index e0995c96..173b99f5 100644
--- a/i386/i386at/model_dep.c
+++ b/i386/i386at/model_dep.c
@@ -129,6 +129,9 @@ extern char	version[];
 /* Realmode temporary GDT */
 extern struct pseudo_descriptor gdt_descr_tmp;
 
+/* Realmode relocated jmp */
+extern uint32_t apboot_jmp_offset;
+
 /* If set, reboot the system on ctrl-alt-delete.  */
 boolean_t	rebootflag = FALSE;	/* exported to kdintr */
 
@@ -221,9 +224,11 @@ void machine_init(void)
 	assert (apboot_addr < 0x100000);
 
 	/*
-	 * Patch the realmode gdt with the correct offset
+	 * Patch the realmode gdt with the correct offset and the first jmp to
+	 * protected mode with the correct target.
 	 */
 	gdt_descr_tmp.linear_base += apboot_addr;
+	apboot_jmp_offset += apboot_addr;
 #endif
 }