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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Sean Christopherson <seanjc@google.com> writes:

> On Fri, Oct 20, 2023, Vitaly Kuznetsov wrote:
>> > ---
>> >  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();
>>  }
>
> That would result in an unnecessray WRMSR in the case where kvmclock is disabled
> on the command line.  It _should_ be benign given how the code is written, but
> it's not impossible to imagine a scenario where someone disabled kvmclock in the
> guest because of a hypervisor bug.  And the WRMSR would become a bogus write to
> MSR 0x0 if someone made a "cleanup" to set msr_kvm_system_time if and only if
> kvmclock is actually used, e.g. if someone made Kirill's change sans the check in
> kvmclock_disable().

True but we don't have such module params to disable other PV features so
e.g. KVM_FEATURE_PV_EOI/KVM_FEATURE_MIGRATION_CONTROL are written to
unconditionally. Wouldn't it be better to handle parameters like
'no-kvmclock' by clearing the feature bit in kvm_arch_para_features()'s
return value so all kvm_para_has_feature() calls for it just return
'false'? We can even do an umbreall "no-kvm-features=<mask>" to cover
all possible debug cases.

-- 
Vitaly