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

[Vadim Rozenfeld <vrozenfe@redhat.com>]

----- 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. 

Peter