Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure

[Roman Kagan]

On Tue, Mar 20, 2018 at 06:35:00PM +0100, Vitaly Kuznetsov wrote:
> Requiring tsc_is_stable_and_known() is too restrictive: even without INVTCS
> nested Hyper-V-on-KVM enables TSC pages for its guests e.g. when
> Reenlightenment MSRs are present. Presence of frequency MSRs doesn't mean
> these frequencies are stable, it just means they're available for reading.
> 
> Signed-off-by: Vitaly Kuznetsov <address@hidden>
> ---
>  target/i386/kvm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
> index 7d9f9ca0b1..74fc3d3b2c 100644
> --- a/target/i386/kvm.c
> +++ b/target/i386/kvm.c
> @@ -651,7 +651,7 @@ 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) {
>              env->features[FEAT_HYPERV_EAX] |= HV_ACCESS_FREQUENCY_MSRS;
>              env->features[FEAT_HYPERV_EDX] |= HV_FREQUENCY_MSRS_AVAILABLE;
>          }

I suggest that we add a corresponding cpu property here, too.  The guest
may legitimately rely on these msrs when it sees the support in CPUID,
and migrating from a kernel with the feature supported (4.14+) to an
older one will make it crash.

Roman.