[PATCH] BUG in pv_clock when overflow condition is detected

[Igor Mammedov <imammedo@redhat.com>]

BUG when overflow occurs at pvclock.c:pvclock_get_nsec_offset

    u64 delta = native_read_tsc() - shadow->tsc_timestamp;

this might happen at an attempt to read an uninitialized yet clock.
It won't prevent stalls and hangs but at least it won't do it silently.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
 arch/x86/kernel/pvclock.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
index 42eb330..35a6190 100644
--- a/arch/x86/kernel/pvclock.c
+++ b/arch/x86/kernel/pvclock.c
@@ -43,7 +43,10 @@ void pvclock_set_flags(u8 flags)
 
 static u64 pvclock_get_nsec_offset(struct pvclock_shadow_time *shadow)
 {
-	u64 delta = native_read_tsc() - shadow->tsc_timestamp;
+	u64 delta;
+	u64 tsc = native_read_tsc();
+	BUG_ON(tsc < shadow->tsc_timestamp);
+	delta = tsc - shadow->tsc_timestamp;
 	return pvclock_scale_delta(delta, shadow->tsc_to_nsec_mul,
 				   shadow->tsc_shift);
 }
-- 
1.7.7.6