Re: [PATCH kvm-unit-tests] KVM: x86: add hyperv clock test case

[Paolo Bonzini <pbonzini@redhat.com>]

On 28/01/2016 15:50, Paolo Bonzini wrote:
> 
> 
> On 28/01/2016 15:25, Andrey Smetanin wrote:
>>>
>>>     Andrey, the test has a relative error of approximately 3 parts
>>>     per million on my machine.  In other words it drifts by 3
>>>     microseconds every second, which I don't think is acceptable.
>>>     The problem is that virtual_tsc_khz is 2593993 while the actual
>>>     frequency is more like 2594001 kHz.
>>>
>>>     But I have a problem in general with using tsc_khz to compute
>>>     the hyperv reference clock scale.  The maximum possible accuracy
>>>     on a 2.5 GHz machine is around 200 ppb, corresponding to a drift
>>>     of about 16 milliseconds per day.  Perhaps we could use Linux
>>>     (and kvmclock's) timekeeping parameters to derive the tsc_scale
>>>     and tsc_offset?
>>
>> Probably need to check the code. But can we just workaround it
>> by host periodic timer with period ~10ms which will issue(by scheduling
>> vcpu KVM_REQ_*) refresh of guest tsc page to prevent drifting?
> 
> That would kill performance.
> 
> However, after a bit more testing, kvmclock seems to have the same
> issue.  The formula to convert the kvmclock scale to the hyper-v one is
> 
> 	tsc_to_nsec_system_mul * 2^(32 + tsc_shift)/100
> 
> which on my system gives 0xFCA52ED99999999; that's within 0.1 ppb of
> what the Hyper-V code computes from a 2593993 kHz TSC.
> 
> So the patch seems okay, but we need to adjust kvm_guest_time_update to
> support the Hyper-V clock.  In kvm_guest_time_update we can compute a
> new scale and offset; if the scale changes, we set sequence to 0.  If
> the new offset is such that the clock would go backwards, we set
> sequence to 0.

... No, that doesn't work.  The clock would go backwards because the
TIME_REF_COUNT MSR is counting more slowly than the TSC-based clock.

I guess what we can do is to compute the TSC reference page parameters
as soon as the master clock is activated, and use them instead of
get_kernel_ns() to compute the reference count MSR (again if the master
clock is activated).  But I'm now a bit wary to enable the hyper-v TSC
reference page without knowing the effects of a divergence between the
two mechanisms.  Especially if a migration could lead to a switch from
one to the other and thus to the time going backwards.

Paolo