Re: [RFC PATCH v2 1/2] add support for Hyper-V reference time counter

[Paolo Bonzini <pbonzini@redhat.com>]

Il 23/05/2013 15:20, Peter Lieven ha scritto:
> On 23.05.2013 15:18, Paolo Bonzini wrote:
>> Il 23/05/2013 14:25, Vadim Rozenfeld ha scritto:
>>>
>>>
>>> ----- Original Message -----
>>> From: "Peter Lieven" <pl@dlhnet.de>
>>> To: "Paolo Bonzini" <pbonzini@redhat.com>
>>> Cc: "Vadim Rozenfeld" <vrozenfe@redhat.com>, "Marcelo Tosatti"
>>> <mtosatti@redhat.com>, kvm@vger.kernel.org, gleb@redhat.com, pl@dlh.net
>>> Sent: Thursday, May 23, 2013 4:17:57 PM
>>> Subject: Re: [RFC PATCH v2 1/2] add support for Hyper-V reference
>>> time counter
>>>
>>> On 22.05.2013 23:55, Paolo Bonzini wrote:
>>>> Il 22/05/2013 09:32, Vadim Rozenfeld ha scritto:
>>>>>>> @@ -1827,6 +1829,29 @@ static int set_msr_hyperv_pw(struct
>>>>>>> kvm_vcpu *vcpu, u32 msr, u64 data)
>>>>>>>            if (__copy_to_user((void __user *)addr, instructions, 4))
>>>>>>>                return 1;
>>>>>>>            kvm->arch.hv_hypercall = data;
>>>>>>> +        local_irq_disable();
>>>>>>> +        kvm->arch.hv_ref_count = get_kernel_ns();
>>>>>>> +        local_irq_enable();
>>>>>>> +        break;
>>>>> local_irq_disable/local_irq_enable not needed.
>>>>>
>>>>>
>>>>> What is the reasoning behind reading this time value at msr write
>>>>> time?
>>>>> [VR] Windows writs this MSR only once, during HAL initialization.
>>>>> So, I decided to treat this call as a partition crate event.
>>>>>
>>>>
>>>> But is it expected by Windows that the reference count starts counting
>>>> up from 0 at partition creation time?  If you could just use
>>>> (get_kernel_ns() + kvm->arch.kvmclock_offset) / 100, it would also be
>>>> simpler for migration purposes.
>>>
>>> I can just report, that I have used the patch that does it that way
>>> and it works.
>>> Maybe Windows is calculating the uptime by the reference counter?
>>>
>>> [VR]
>>> Windows use it (reference counters/iTSC/PMTimer/HPET) as a time-stamp
>>> source
>>> for (Ke)QueryPerformanceCounter function.
>>
>> So I would prefer to remove kvm->arch.hv_ref_count altogether.
> 
> But only if the migration support is guaranteed.

Migration support wouldn't work yet anyway, you need to recompute the
scale and sequence.  But that could be done by KVM_SET_CLOCK.

> And what if we have a host which lacks invariant TSC support?

Then the sequence must be set to 0 or 0xFFFFFFFF, I still haven't
understood. :)

Paolo