KVM: x86: workaround SuSE's 2.6.16 pvclock vs masterclock issue

SuSE's 2.6.16 kernel fails to boot if the delta between tsc_timestamp
and rdtsc is larger than a given threshold:

 * If we get more than the below threshold into the future, we rerequest
 * the real time from the host again which has only little offset then
 * that we need to adjust using the TSC.
 *
 * For now that threshold is 1/5th of a jiffie. That should be good
 * enough accuracy for completely broken systems, but also give us swing
 * to not call out to the host all the time.
 */
#define PVCLOCK_DELTA_MAX ((1000000000ULL / HZ) / 5)

Disable masterclock support (which increases said delta) in case the
boot vcpu does not use MSR_KVM_SYSTEM_TIME_NEW.

Upstreams kernels which support pvclock vsyscalls (and therefore make
use of PVCLOCK_STABLE_BIT) use MSR_KVM_SYSTEM_TIME_NEW.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -627,6 +627,8 @@ struct kvm_arch {
 	#ifdef CONFIG_KVM_MMU_AUDIT
 	int audit_point;
 	#endif
+
+	bool boot_vcpu_runs_old_kvmclock;
 };
 
 struct kvm_vm_stat {

arch/x86/kvm/x86.c

@@ -1542,7 +1542,8 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
 					&ka->master_cycle_now);
 
 	ka->use_master_clock = host_tsc_clocksource && vcpus_matched
-				&& !backwards_tsc_observed;
+				&& !backwards_tsc_observed
+				&& !ka->boot_vcpu_runs_old_kvmclock;
 
 	if (ka->use_master_clock)
 		atomic_set(&kvm_guest_has_master_clock, 1);
@@ -2174,8 +2175,20 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	case MSR_KVM_SYSTEM_TIME_NEW:
 	case MSR_KVM_SYSTEM_TIME: {
 		u64 gpa_offset;
+		struct kvm_arch *ka = &vcpu->kvm->arch;
+
 		kvmclock_reset(vcpu);
 
+		if (vcpu->vcpu_id == 0 && !msr_info->host_initiated) {
+			bool tmp = (msr == MSR_KVM_SYSTEM_TIME);
+
+			if (ka->boot_vcpu_runs_old_kvmclock != tmp)
+				set_bit(KVM_REQ_MASTERCLOCK_UPDATE,
+					&vcpu->requests);
+
+			ka->boot_vcpu_runs_old_kvmclock = tmp;
+		}
+
 		vcpu->arch.time = data;
 		kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);