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Re: [Qemu-devel] [PATCH v4 2/2] i386/kvm: expose Hyper-V frequency MSRs
From: |
Vitaly Kuznetsov |
Subject: |
Re: [Qemu-devel] [PATCH v4 2/2] i386/kvm: expose Hyper-V frequency MSRs with reenlightenment |
Date: |
Fri, 23 Mar 2018 15:45:49 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux) |
Eduardo Habkost <address@hidden> 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 <address@hidden>
>> ---
>> - 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