Roman Kagan <rka...@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 <vkuzn...@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
