x86: kvmclock: Do not setup kvmclock vsyscall in the absence of that clock

This fixes boot lockups with "no-kvmclock", when the host is not
exposing this particular feature (QEMU: -cpu ...,-kvmclock) or when
the kvmclock initialization failed for whatever reason.
Reviewed-by: NMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NGleb Natapov <gleb@redhat.com>

arch/x86/kernel/kvmclock.c

@@ -160,8 +160,12 @@ int kvm_register_clock(char *txt)
 {
 	int cpu = smp_processor_id();
 	int low, high, ret;
-	struct pvclock_vcpu_time_info *src = &hv_clock[cpu].pvti;
+	struct pvclock_vcpu_time_info *src;
+
+	if (!hv_clock)
+		return 0;
 
+	src = &hv_clock[cpu].pvti;
 	low = (int)__pa(src) | 1;
 	high = ((u64)__pa(src) >> 32);
 	ret = native_write_msr_safe(msr_kvm_system_time, low, high);
@@ -276,6 +280,9 @@ int __init kvm_setup_vsyscall_timeinfo(void)
 	struct pvclock_vcpu_time_info *vcpu_time;
 	unsigned int size;
 
+	if (!hv_clock)
+		return 0;
+
 	size = PAGE_ALIGN(sizeof(struct pvclock_vsyscall_time_info)*NR_CPUS);
 
 	preempt_disable();