rtc: map CMOS index 0x37 to 0x32 on read and writes

QEMU's attempt to implement the century byte cover two possible places
for the byte.  A common one on modern chipsets is 0x32, but QEMU also
stores the value in 0x37 (apparently for IBM PS/2 compatibility---it's
only been 25 years).  To simplify the implementation of the century
byte, store it only at 0x32 but remap transparently 0x37 to 0x32 when
reading and writing from CMOS.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/mc146818rtc.c

@@ -399,6 +399,10 @@ static void cmos_ioport_write(void *opaque, uint32_t addr, uint32_t data)
             s->cmos_data[s->cmos_index] = data;
             check_update_timer(s);
             break;
+	case RTC_IBM_PS2_CENTURY_BYTE:
+            s->cmos_index = RTC_CENTURY;
+            /* fall through */
+        case RTC_CENTURY:
         case RTC_SECONDS:
         case RTC_MINUTES:
         case RTC_HOURS:
@@ -598,6 +602,10 @@ static uint32_t cmos_ioport_read(void *opaque, uint32_t addr)
         return 0xff;
     } else {
         switch(s->cmos_index) {
+	case RTC_IBM_PS2_CENTURY_BYTE:
+            s->cmos_index = RTC_CENTURY;
+            /* fall through */
+        case RTC_CENTURY:
         case RTC_SECONDS:
         case RTC_MINUTES:
         case RTC_HOURS:
@@ -661,10 +669,6 @@ void rtc_set_memory(ISADevice *dev, int addr, int val)
         s->cmos_data[addr] = val;
 }
 
-/* PC cmos mappings */
-#define REG_IBM_CENTURY_BYTE        0x32
-#define REG_IBM_PS2_CENTURY_BYTE    0x37
-
 static void rtc_set_date_from_host(ISADevice *dev)
 {
     RTCState *s = DO_UPCAST(RTCState, dev, dev);
@@ -681,8 +685,7 @@ static void rtc_set_date_from_host(ISADevice *dev)
     rtc_set_cmos(s, &tm);
 
     val = rtc_to_bcd(s, (tm.tm_year / 100) + 19);
-    rtc_set_memory(dev, REG_IBM_CENTURY_BYTE, val);
-    rtc_set_memory(dev, REG_IBM_PS2_CENTURY_BYTE, val);
+    rtc_set_memory(dev, RTC_CENTURY, val);
 }
 
 static int rtc_post_load(void *opaque, int version_id)

hw/mc146818rtc_regs.h

@@ -44,6 +44,10 @@
 #define RTC_REG_C               12
 #define RTC_REG_D               13
 
+/* PC cmos mappings */
+#define RTC_CENTURY              0x32
+#define RTC_IBM_PS2_CENTURY_BYTE 0x37
+
 #define REG_A_UIP 0x80
 
 #define REG_B_SET  0x80