Re: [Qemu-devel] [PATCH 3/3] i386/kvm: advertise Hyper-V frequency MSRs

[Ladi Prosek]

On Fri, Aug 4, 2017 at 3:39 PM, David Hildenbrand <address@hidden> wrote:
> On 04.08.2017 11:14, Ladi Prosek wrote:
>> As of kernel commit eb82feea59d6 ("KVM: hyperv: support 
>> HV_X64_MSR_TSC_FREQUENCY
>> and HV_X64_MSR_APIC_FREQUENCY"), KVM supports two new MSRs which are required
>> for nested Hyper-V to read timestamps with RDTSC + TSC page.
>>
>> This commit makes QEMU advertise the MSRs with CPUID.40000003H:EAX[11] and
>> CPUID.40000003H:EDX[8] as specified in the Hyper-V TLFS and experimentally
>> verified on a Hyper-V host. The feature is enabled with the existing hv-time 
>> CPU
>> flag, and only if the TSC frequency is stable across migration and known.
>>
>> Signed-off-by: Ladi Prosek <address@hidden>
>> ---
>>  target/i386/kvm.c | 16 +++++++++++++++-
>>  1 file changed, 15 insertions(+), 1 deletion(-)
>>
>> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
>> index 77b6373..7e484a7 100644
>> --- a/target/i386/kvm.c
>> +++ b/target/i386/kvm.c
>> @@ -89,6 +89,7 @@ static bool has_msr_hv_vpindex;
>>  static bool has_msr_hv_runtime;
>>  static bool has_msr_hv_synic;
>>  static bool has_msr_hv_stimer;
>> +static bool has_msr_hv_frequencies;
>>  static bool has_msr_xss;
>>
>>  static bool has_msr_architectural_pmu;
>> @@ -631,7 +632,17 @@ static int hyperv_handle_properties(CPUState *cs)
>>      if (cpu->hyperv_time) {
>>          env->features[FEAT_HYPERV_EAX] |= HV_X64_MSR_HYPERCALL_AVAILABLE;
>>          env->features[FEAT_HYPERV_EAX] |= 
>> HV_X64_MSR_TIME_REF_COUNT_AVAILABLE;
>> -        env->features[FEAT_HYPERV_EAX] |= 0x200;
>> +        env->features[FEAT_HYPERV_EAX] |= 
>> HV_X64_MSR_REFERENCE_TSC_AVAILABLE;
>> +        if (has_msr_hv_frequencies> +            /* TSC clock must be 
>> stable and known for this feature. */
>> +            && ((env->features[FEAT_8000_0007_EDX] & CPUID_APM_INVTSC)
>> +                || env->user_tsc_khz != 0)
>> +            && env->tsc_khz != 0) {
>
> I'd drop the != 0 in both cases and move the env->tsc_khz check up to
> has_msr_hv_frequencies.
>
> if (has_msr_hv_frequencies && env->tsc_khz && ...
>
> Wonder if it even would make sense to move some parts of this check into
> a helper function, to beautify this a bit. tsc_stable(env)
> tsc_known(env) ...

Yup, especially since I copied these conditions from "if
(cpu->vmware_cpuid_freq .." further down in the file. So maybe one
helper called by both would be the best. Thanks!

>> +
>> +            env->features[FEAT_HYPERV_EAX] |= HV_X64_ACCESS_FREQUENCY_MSRS;
>> +            env->features[FEAT_HYPERV_EDX] |=
>> +                HV_FEATURE_FREQUENCY_MSRS_AVAILABLE;
>> +        }
>>      }
>>      if (cpu->hyperv_crash && has_msr_hv_crash) {
>>          env->features[FEAT_HYPERV_EDX] |= HV_X64_GUEST_CRASH_MSR_AVAILABLE;
>> @@ -1127,6 +1138,9 @@ static int kvm_get_supported_msrs(KVMState *s)
>>                  case HV_X64_MSR_STIMER0_CONFIG:
>>                      has_msr_hv_stimer = true;
>>                      break;
>> +                case HV_X64_MSR_TSC_FREQUENCY:
>> +                    has_msr_hv_frequencies = true;
>> +                    break;
>>                  }
>>
>>              }
>>
>
>
> --
>
> Thanks,
>
> David