goldfish_rtc: Fix non-atomic read behaviour of TIME_LOW/TIME_HIGH

The specification says:

   0x00  TIME_LOW   R: Get current time, then return low-order 32-bits.
   0x04  TIME_HIGH  R: Return high 32-bits from previous TIME_LOW read.

   ...

   To read the value, the kernel must perform an IO_READ(TIME_LOW),
   which returns an unsigned 32-bit value, before an IO_READ(TIME_HIGH),
   which returns a signed 32-bit value, corresponding to the higher half
   of the full value.

However, we were just returning the current time for both. If the guest
is unlucky enough to read TIME_LOW and TIME_HIGH either side of an
overflow of the lower half, it will see time be in the future, before
jumping backwards on the next read, and Linux currently relies on the
atomicity guaranteed by the spec so is affected by this. Fix this
violation of the spec by caching the correct value for TIME_HIGH
whenever TIME_LOW is read, and returning that value for any TIME_HIGH
read.

Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200718004934.83174-1-jrtc27@jrtc27.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

hw/rtc/goldfish_rtc.c

@@ -94,12 +94,22 @@ static uint64_t goldfish_rtc_read(void *opaque, hwaddr offset,
     GoldfishRTCState *s = opaque;
     uint64_t r = 0;
 
+    /*
+     * From the documentation linked at the top of the file:
+     *
+     *   To read the value, the kernel must perform an IO_READ(TIME_LOW), which
+     *   returns an unsigned 32-bit value, before an IO_READ(TIME_HIGH), which
+     *   returns a signed 32-bit value, corresponding to the higher half of the
+     *   full value.
+     */
     switch (offset) {
     case RTC_TIME_LOW:
-        r = goldfish_rtc_get_count(s) & 0xffffffff;
+        r = goldfish_rtc_get_count(s);
+        s->time_high = r >> 32;
+        r &= 0xffffffff;
         break;
     case RTC_TIME_HIGH:
-        r = goldfish_rtc_get_count(s) >> 32;
+        r = s->time_high;
         break;
     case RTC_ALARM_LOW:
         r = s->alarm_next & 0xffffffff;
@@ -216,7 +226,7 @@ static const MemoryRegionOps goldfish_rtc_ops = {
 
 static const VMStateDescription goldfish_rtc_vmstate = {
     .name = TYPE_GOLDFISH_RTC,
-    .version_id = 1,
+    .version_id = 2,
     .pre_save = goldfish_rtc_pre_save,
     .post_load = goldfish_rtc_post_load,
     .fields = (VMStateField[]) {
@@ -225,6 +235,7 @@ static const VMStateDescription goldfish_rtc_vmstate = {
         VMSTATE_UINT32(alarm_running, GoldfishRTCState),
         VMSTATE_UINT32(irq_pending, GoldfishRTCState),
         VMSTATE_UINT32(irq_enabled, GoldfishRTCState),
+        VMSTATE_UINT32(time_high, GoldfishRTCState),
         VMSTATE_END_OF_LIST()
     }
 };

include/hw/rtc/goldfish_rtc.h

@@ -41,6 +41,7 @@ typedef struct GoldfishRTCState {
     uint32_t alarm_running;
     uint32_t irq_pending;
     uint32_t irq_enabled;
+    uint32_t time_high;
 } GoldfishRTCState;
 
 #endif