Re: [Qemu-devel] [PATCH v4 2/2] i386/kvm: expose Hyper-V frequency MSRs with reenlightenment

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Thu, Mar 22, 2018 at 02:13:58PM +0100, Vitaly Kuznetsov wrote:
>> We can also expose Hyper-V frequency MSRs when reenlightenment feature is
>> enabled and TSC frequency is known, Hyper-V on KVM will provide stable TSC
>> page clocksources to its guests.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>> - Expose frequency MSRs only when either INVTSC or Reenlightenment is
>>   provided [Paolo Bonzini]
>> ---
>>  target/i386/kvm.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>> 
>> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
>> index 75f4e1d69e..2c3c19d690 100644
>> --- a/target/i386/kvm.c
>> +++ b/target/i386/kvm.c
>> @@ -651,7 +651,8 @@ static int hyperv_handle_properties(CPUState *cs)
>>          env->features[FEAT_HYPERV_EAX] |= HV_TIME_REF_COUNT_AVAILABLE;
>>          env->features[FEAT_HYPERV_EAX] |= HV_REFERENCE_TSC_AVAILABLE;
>>  
>> -        if (has_msr_hv_frequencies && tsc_is_stable_and_known(env)) {
>> +        if (has_msr_hv_frequencies && env->tsc_khz &&
>
> Why is the check for env->tsc_khz necessary?
>
> Are there known circumstances where HV_X64_MSR_TSC_FREQUENCY will be supported
> by KVM but ioctl(KVM_GET_TSC_KHZ) will return 0, or this is just for extra
> safety?
>

Yes,

I didn't experiment with passing '0' to Windows but in general it
doesn't sound like a good idea. 

>> +            (tsc_is_stable_and_known(env) || cpu->hyperv_reenlightenment)) {
>>              env->features[FEAT_HYPERV_EAX] |= HV_ACCESS_FREQUENCY_MSRS;
>>              env->features[FEAT_HYPERV_EDX] |= HV_FREQUENCY_MSRS_AVAILABLE;
>>          }
>> -- 
>> 2.14.3
>> 

-- 
  Vitaly