kvm: x86: request masterclock update any time guest uses different msr

Commit 5b9bb0eb ("kvm: x86: encapsulate wrmsr(MSR_KVM_SYSTEM_TIME)
emulation in helper fn", 2020-10-21) subtly changed the behavior of guest
writes to MSR_KVM_SYSTEM_TIME(_NEW). Restore the previous behavior; update
the masterclock any time the guest uses a different msr than before.

Fixes: 5b9bb0eb ("kvm: x86: encapsulate wrmsr(MSR_KVM_SYSTEM_TIME) emulation in helper fn", 2020-10-21)
Signed-off-by: NOliver Upton <oupton@google.com>
Reviewed-by: NPeter Shier <pshier@google.com>
Message-Id: <20201027231044.655110-6-oupton@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -1967,7 +1967,7 @@ static void kvm_write_system_time(struct kvm_vcpu *vcpu, gpa_t system_time,
 	struct kvm_arch *ka = &vcpu->kvm->arch;
 
 	if (vcpu->vcpu_id == 0 && !host_initiated) {
-		if (ka->boot_vcpu_runs_old_kvmclock && old_msr)
+		if (ka->boot_vcpu_runs_old_kvmclock != old_msr)
 			kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);
 
 		ka->boot_vcpu_runs_old_kvmclock = old_msr;