s390x/tcg: avoid overflows in time2tod/tod2time

Big values for the TOD/ns clock can result in some overflows that can be
avoided. Not all overflows can be handled however, as the conversion either
multiplies by 4.096 or divided by 4.096.

Apply the trick used in the Linux kernel in arch/s390/include/asm/timex.h
for tod_to_ns() and use the same trick also for the conversion in the
other direction.
Reviewed-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NDavid Hildenbrand <david@redhat.com>
Message-Id: <20180627134410.4901-2-david@redhat.com>
Signed-off-by: NCornelia Huck <cohuck@redhat.com>

target/s390x/internal.h

@@ -243,13 +243,14 @@ enum cc_op {
 /* Converts ns to s390's clock format */
 static inline uint64_t time2tod(uint64_t ns)
 {
-    return (ns << 9) / 125;
+    return (ns << 9) / 125 + (((ns & 0xff10000000000000ull) / 125) << 9);
+
 }
 
 /* Converts s390's clock format to ns */
 static inline uint64_t tod2time(uint64_t t)
 {
-    return (t * 125) >> 9;
+    return ((t >> 9) * 125) + (((t & 0x1ff) * 125) >> 9);
 }
 
 static inline hwaddr decode_basedisp_s(CPUS390XState *env, uint32_t ipb,