1 files changed, 16 insertions, 2 deletions

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 1e6244202612..24249a5360b6 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -767,6 +767,7 @@ static cycle_t read_tsc(struct clocksource *cs)
 static cycle_t __vsyscall_fn vread_tsc(void)
 {
         cycle_t ret;
+        u64 last;
 
         /*
          * Empirically, a fence (of type that depends on the CPU)
@@ -778,8 +779,21 @@ static cycle_t __vsyscall_fn vread_tsc(void)
         rdtsc_barrier();
         ret = (cycle_t)vget_cycles();
 
-        return ret >= VVAR(vsyscall_gtod_data).clock.cycle_last ?
-                ret : VVAR(vsyscall_gtod_data).clock.cycle_last;
+        last = VVAR(vsyscall_gtod_data).clock.cycle_last;
+
+        if (likely(ret >= last))
+                return ret;
+
+        /*
+         * GCC likes to generate cmov here, but this branch is extremely
+         * predictable (it's just a funciton of time and the likely is
+         * very likely) and there's a data dependence, so force GCC
+         * to generate a branch instead.  I don't barrier() because
+         * we don't actually need a barrier, and if this function
+         * ever gets inlined it will generate worse code.
+         */
+        asm volatile ("");
+        return last;
 }
 #endif