boot: Don't jump back to protected mode in EFI loader

boot/multiboot.c

@@ -264,39 +264,9 @@ static void finish_boot(void) {
 		}
 	}
 
-	uint64_t foobar = ((uint32_t)(uintptr_t)&do_the_nasty) | (0x10L << 32L);
-
-	uint32_t * foo = (uint32_t *)0x7c00;
-
-	foo[0] = MULTIBOOT_EAX_MAGIC;
-	foo[1] = (uintptr_t)finalHeader;
-	foo[2] = _xmain;
-
+	/* Jump to entry with register arguments */
 	__asm__ __volatile__ (
-			"push %0\n"
-			"lretl\n"
-			 : : "g"(foobar));
-
-	__asm__ (
-		"do_the_nasty:\n"
-		"cli\n"
-		".code32\n"
-		"mov %cr0, %eax\n"
-		"and $0x7FFeFFFF, %eax\n"
-		"mov %eax, %cr0\n"
-		"mov $0xc0000080, %ecx\n"
-		"rdmsr\n"
-		"and $0xfffffeff, %eax\n"
-		"wrmsr\n"
-		"mov $0x640, %eax\n"
-		"mov %eax, %cr4\n"
-		"mov 0x7c00, %eax\n"
-		"mov 0x7c04, %ebx\n"
-		"mov 0x7c08, %ecx\n"
-		"jmp *%ecx\n"
-		"target: jmp target\n"
-		".code64\n"
-		);
+		"jmp %0" :: "r"(_xmain), "a"(MULTIBOOT_EAX_MAGIC), "b"(finalHeader));
 
 	__builtin_unreachable();
 }

kernel/arch/x86_64/bootstrap.S

@@ -151,7 +151,6 @@ jmp_to_long:
 
     /* Set up initial page region, which was zero'd for us by the loader */
     mov $init_page_region, %edi
-    mov %edi, %cr3
 
     /* PML4[0] = &PDP[0] | (PRESENT, WRITABLE, USER) */
     mov $0x1007, %eax
@@ -179,6 +178,9 @@ jmp_to_long:
     add $8, %edi
     loop .set_entry
 
+    mov $init_page_region, %edi
+    mov %edi, %cr3
+
     /* Enable PAE */
     mov %cr4, %eax
     or $32, %eax
@@ -190,12 +192,19 @@ jmp_to_long:
     or $256, %eax
     wrmsr
 
-    /* Set PG */
+    /* Check PG */
     mov %cr0, %eax
+
+    /* If paging was enabled, assume we were already in long mode (eg. booted by EFI) */
+    test $0x80000000, %eax
+    jnz .continue
+
+    /* Otherwise enable paging */
     or $0x80000000, %eax
     mov %eax, %cr0
 
     lgdt gdtr
+
     ljmp $0x08,$realm64
 
 .align 8
@@ -236,6 +245,8 @@ realm64:
     mov %ax, %gs
     mov %ax, %ss
 
+.continue:
+    cli
     pop %rdi
     pop %rsi
     pop %rdx