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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Roman Kagan <rkagan@virtuozzo.com> writes:

> 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 <vkuznets@redhat.com>
>> ---
>>  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.
>

This can be arranged, but what happens to people who use these features
today? Assuming they also passed 'invtsc' they have stable TSC page
clocksource already (when Hyper-V role is enabled) but when we start
requesting a new 'hv_frequency' cpu property they'll suddenly lose what
they have...

-- 
  Vitaly