summaryrefslogtreecommitdiff
diff options
context:
space:
mode:
authorHeiko Carstens <heiko.carstens@de.ibm.com>2013-01-14 16:55:55 +0100
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2013-01-21 11:45:15 -0800
commit2017a7c8ee63e678c20479e24416aac20a8899dc (patch)
treeb3d419ccba3675c59a2042547a728f0ea9a83493
parentaa16959322c2c096ac050d8a970e1cf2d2b1cc35 (diff)
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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--arch/s390/include/asm/timex.h28
-rw-r--r--arch/s390/kernel/time.c2
-rw-r--r--arch/s390/kvm/interrupt.c2
3 files changed, 30 insertions, 2 deletions
diff --git a/arch/s390/include/asm/timex.h b/arch/s390/include/asm/timex.h
index c447a27a7fdb..945b7cdf4934 100644
--- 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 d4e1cb1dbcd1..c5531dbc8b2f 100644
--- 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 2d9f9a72bb81..10e13b331d38 100644
--- a/arch/s390/kvm/interrupt.c
+++ b/arch/s390/kvm/interrupt.c
@@ -390,7 +390,7 @@ 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);