x86/kvm: Do not try to disable kvmclock if it was not enabled

kvm_guest_cpu_offline() tries to disable kvmclock regardless if it is
present in the VM. It leads to write to a MSR that doesn't exist on some
configurations, namely in TDX guest:

	unchecked MSR access error: WRMSR to 0x12 (tried to write 0x0000000000000000)
	at rIP: 0xffffffff8110687c (kvmclock_disable+0x1c/0x30)

kvmclock enabling is gated by CLOCKSOURCE and CLOCKSOURCE2 KVM paravirt
features.

Do not disable kvmclock if it was not enabled.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Fixes: c02027b ("x86/kvm: Disable kvmclock on all CPUs on shutdown")
Reviewed-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Wanpeng Li <wanpengli@tencent.com>
Cc: stable@vger.kernel.org
Message-Id: <20231205004510.27164-6-kirill.shutemov@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kernel/kvmclock.c

@@ -24,8 +24,8 @@
 
 static int kvmclock __initdata = 1;
 static int kvmclock_vsyscall __initdata = 1;
-static int msr_kvm_system_time __ro_after_init = MSR_KVM_SYSTEM_TIME;
-static int msr_kvm_wall_clock __ro_after_init = MSR_KVM_WALL_CLOCK;
+static int msr_kvm_system_time __ro_after_init;
+static int msr_kvm_wall_clock __ro_after_init;
 static u64 kvm_sched_clock_offset __ro_after_init;
 
 static int __init parse_no_kvmclock(char *arg)
@@ -195,7 +195,8 @@ static void kvm_setup_secondary_clock(void)
 
 void kvmclock_disable(void)
 {
-	native_write_msr(msr_kvm_system_time, 0, 0);
+	if (msr_kvm_system_time)
+		native_write_msr(msr_kvm_system_time, 0, 0);
 }
 
 static void __init kvmclock_init_mem(void)
@@ -294,7 +295,10 @@ void __init kvmclock_init(void)
 	if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE2)) {
 		msr_kvm_system_time = MSR_KVM_SYSTEM_TIME_NEW;
 		msr_kvm_wall_clock = MSR_KVM_WALL_CLOCK_NEW;
-	} else if (!kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE)) {
+	} else if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE)) {
+		msr_kvm_system_time = MSR_KVM_SYSTEM_TIME;
+		msr_kvm_wall_clock = MSR_KVM_WALL_CLOCK;
+	} else {
 		return;
 	}