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

[Eduardo Habkost <ehabkost@redhat.com>]

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 &&
> +            (tsc_is_stable_and_known(env) || cpu->hyperv_reenlightenment)) {

Continuing the discussion we had in v3 about being possible to
crash when migrating to a host running an older kernel:

This patch doesn't fix the issue, because it is still implicitly
enabling hv-frequencies.

But I don't think this patch will make the situation any worse,
because we don't have any KVM versions that support
HV_X64_MSR_REENLIGHTENMENT_CONTROL but not
HV_X64_MSR_TSC_FREQUENCY.

This means that we can safely make "hv-reenlightenment=on"
automatically set "hv-frequencies=on", when we finally introduce
a hv-frequencies property.

Roman, do you agree?

The next question is: do we need to fix this and introduce a
hv-frequencies property in 2.12, or can this wait for 2.13?


>              env->features[FEAT_HYPERV_EAX] |= HV_ACCESS_FREQUENCY_MSRS;
>              env->features[FEAT_HYPERV_EDX] |= HV_FREQUENCY_MSRS_AVAILABLE;
>          }
> -- 
> 2.14.3
> 
> 

-- 
Eduardo