[pcbios] Don't use "lret $2" to return from an interrupt

Using "lret $2" to return from an interrupt causes interrupts to be
disabled in the calling program, since the INT instruction will have
disabled interrupts.  Instead, patch CF on the stack and use iret to
return.

Interestingly, the original PC BIOS had this bug in at least one
place.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Michael Brown <mcb30@etherboot.org>

src/arch/i386/firmware/pcbios/fakee820.c

@@ -63,6 +63,8 @@ void fake_e820 ( void ) {
 			      "cmpl $0x534d4150, %%edx\n\t"
 			      "jne 99f\n\t"
 			      "pushaw\n\t"
+			      "movw %%sp, %%bp\n\t"
+			      "andb $~0x01, 22(%%bp)\n\t" /* Clear return CF */
 			      "leaw e820map(%%bx), %%si\n\t"
 			      "cs rep movsb\n\t"
 			      "popaw\n\t"
@@ -73,8 +75,7 @@ void fake_e820 ( void ) {
 			      "xorl %%ebx,%%ebx\n\t"
 			      "\n1:\n\t"
 			      "popfw\n\t"
-			      "clc\n\t"
-			      "lret $2\n\t"
+			      "iret\n\t"
 			      "\n99:\n\t"
 			      "popfw\n\t"
 			      "ljmp *%%cs:real_int15_vector\n\t" )