Re: [RFC PATCH v3 2/2] add support for Hyper-V partition reference time enlightenment

[Paolo Bonzini <pbonzini@redhat.com>]

Il 11/12/2013 11:58, Vadim Rozenfeld ha scritto:
>> > +			curr_time = (((tsc_ref->tsc_scale >> 32) * native_read_tsc()) >> 32) + 
>> > +				tsc_ref->tsc_offset;
>> > +			tsc_ref->tsc_offset = kvm->arch.hv_ref_time - curr_time;
>> > 
>> > Why do you need kvm->arch.hv_ref_time at all?  Can you just use
>> > "get_kernel_ns() + kvm->arch.kvmclock_offset - kvm->arch.hv_ref_count"?
>> > Then the same code can set tsc_ref->tsc_offset in both cases.
>> > 
>> > In fact, it's not clear to me what hv_ref_time is for, and how it
>> > is different from 
> OK, let me explain how it works.
> Hyper-V allows guest to use invariant TSC provided by host as a time
> stamp source (KeQueryPerformanceCounter). Guest is calling rdtsc and
> normalizing it to 10MHz frequency, it is why we need "tsc_scale".
> "tsc_offset" is needed for migration or pause/resume cycles.
> When we pause a VM, we need to save the current vTSC value
> ("hv_ref_time"), which is rdtsc * tsc_scale + tsc_offset.
> Then, during resume, we need to recalculate the new tsc_scale
> as well as the new tsc_offset value. 
> tsc_offset = old(saved) vTSC - new vTSC

In practice "save" means KVM_GET_CLOCK, and "restore" means
KVM_SET_CLOCK, right?

> So maybe hv_ref_time is not a good name, but we use it 
> for keeping the old vTSC value, saved before stopping VM.

Ok, this was roughly my understanding as well.

My understanding is also that (((tsc_ref->tsc_scale >> 32) *
native_read_tsc()) >> 32) + tsc_ref->tsc_offset returns exactly the same
value as HV_X64_MSR_TIME_REF_COUNT.  Thus we do not need
kvm->arch.hv_ref_time.  We can use the value of
HV_X64_MSR_TIME_REF_COUNT, which is "(get_kernel_ns() +
kvm->arch.kvmclock_offset - kvm->arch.hv_ref_count) / 100", to compute
tsc_offset, like this:

  curr_time = (((tsc_ref->tsc_scale >> 32) * native_read_tsc()) >> 32);
  tsc_ref->tsc_offset = get_hv_x64_msr_time_ref_count() - curr_time;

This code can be applied always: when the TSC page is initialized and
when KVM_SET_CLOCK is called.  You do not need to do anything for
KVM_GET_CLOCK.

Paolo