Re: [PATCHv2 04/13] x86/kvm: Do not try to disable kvmclock if it was not enabled

[Vitaly Kuznetsov <vkuznets@redhat.com>]

"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> writes:

> 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: c02027b5742b ("x86/kvm: Disable kvmclock on all CPUs on shutdown")
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Wanpeng Li <wanpengli@tencent.com>
> Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
> Cc: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kernel/kvmclock.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index fb8f52149be9..f2fff625576d 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/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;
>  	}

This should work, so

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

but my personal preference would be to change kvm_guest_cpu_offline()
to check KVM features explicitly instead of checking MSRs against '0'
at least becase it already does so for other features. Completely
untested:

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index b8ab9ee5896c..1ee49c98e70a 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -454,7 +454,9 @@ static void kvm_guest_cpu_offline(bool shutdown)
        kvm_pv_disable_apf();
        if (!shutdown)
                apf_task_wake_all();
-       kvmclock_disable();
+       if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE2) ||
+           kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE))
+               kvmclock_disable();
 }

-- 
Vitaly