s390/time: fix sched_clock() overflow

commit ed4f20943cd4c7b55105c04daedf8d63ab6d499c upstream.

Converting a 64 Bit TOD format value to nanoseconds means that the value
must be divided by 4.096. In order to achieve that we multiply with 125
and divide by 512.
When used within sched_clock() this triggers an overflow after appr.
417 days. Resulting in a sched_clock() return value that is much smaller
than previously and therefore may cause all sort of weird things in
subsystems that rely on a monotonic sched_clock() behaviour.

To fix this implement a tod_to_ns() helper function which converts TOD
values without overflow and call this function from both places that
open coded the conversion: sched_clock() and kvm_s390_handle_wait().

Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/arch/s390/include/asm/timex.h b/arch/s390/include/asm/timex.h
index c447a27a7fdb109be95edf0d6884026df6e895ce..945b7cdf4934261e80e73bdc81f4ff6e9b617064 100644 (file)
--- a/arch/s390/include/asm/timex.h
+++ b/arch/s390/include/asm/timex.h
@@ -137,4 +137,32 @@ static inline unsigned long long get_clock_monotonic(void)
        return get_clock_xt() - sched_clock_base_cc;
 }
 
+/**
+ * tod_to_ns - convert a TOD format value to nanoseconds
+ * @todval: to be converted TOD format value
+ * Returns: number of nanoseconds that correspond to the TOD format value
+ *
+ * Converting a 64 Bit TOD format value to nanoseconds means that the value
+ * must be divided by 4.096. In order to achieve that we multiply with 125
+ * and divide by 512:
+ *
+ *    ns = (todval * 125) >> 9;
+ *
+ * In order to avoid an overflow with the multiplication we can rewrite this.
+ * With a split todval == 2^32 * th + tl (th upper 32 bits, tl lower 32 bits)
+ * we end up with
+ *
+ *    ns = ((2^32 * th + tl) * 125 ) >> 9;
+ * -> ns = (2^23 * th * 125) + ((tl * 125) >> 9);
+ *
+ */
+static inline unsigned long long tod_to_ns(unsigned long long todval)
+{
+       unsigned long long ns;
+
+       ns = ((todval >> 32) << 23) * 125;
+       ns += ((todval & 0xffffffff) * 125) >> 9;
+       return ns;
+}
+
 #endif

diff --git a/arch/s390/kernel/time.c b/arch/s390/kernel/time.c
index e03c55525823cde2e56493791a8635cd4837bd3b..86443663e1bbef3536da62c9a3f99d0dcf1d77d5 100644 (file)
--- a/arch/s390/kernel/time.c
+++ b/arch/s390/kernel/time.c
@@ -64,7 +64,7 @@ static DEFINE_PER_CPU(struct clock_event_device, comparators);
  */
 unsigned long long notrace __kprobes sched_clock(void)
 {
-       return (get_clock_monotonic() * 125) >> 9;
+       return tod_to_ns(get_clock_monotonic());
 }
 
 /*

diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
index 278ee009ce6570a9d4049a095def51b17fb164d5..5482d1ea65025b29d528a74029be38ef7d7455c5 100644 (file)
--- a/arch/s390/kvm/interrupt.c
+++ b/arch/s390/kvm/interrupt.c
@@ -391,7 +391,7 @@ int kvm_s390_handle_wait(struct kvm_vcpu *vcpu)
                return 0;
        }
 
-       sltime = ((vcpu->arch.sie_block->ckc - now)*125)>>9;
+       sltime = tod_to_ns(vcpu->arch.sie_block->ckc - now);
 
        hrtimer_start(&vcpu->arch.ckc_timer, ktime_set (0, sltime) , HRTIMER_MODE_REL);
        VCPU_EVENT(vcpu, 5, "enabled wait via clock comparator: %llx ns", sltime);