On Tue, 2013-12-10 at 17:52 +0100, Paolo Bonzini wrote:
> Il 10/12/2013 12:23, Vadim Rozenfeld ha scritto:
> > > > + if (kvm->arch.hv_tsc_page & HV_X64_MSR_TSC_REFERENCE_ENABLE) {
> > > > + HV_REFERENCE_TSC_PAGE* tsc_ref;
> > > > + u64 curr_time;
> > > > + tsc_ref = (HV_REFERENCE_TSC_PAGE*)gfn_to_hva(kvm,
> > > > + kvm->arch.hv_tsc_page >> HV_X64_MSR_TSC_REFERENCE_ADDRESS_SHIFT);
> > > > + tsc_ref->tsc_sequence =
> > > > + boot_cpu_has(X86_FEATURE_CONSTANT_TSC) ? tsc_ref->tsc_sequence + 1 : 0;
> > > > + tsc_ref->tsc_scale = ((10000LL << 32) / __get_cpu_var(cpu_tsc_khz)) << 32;
> > >
> > > Why shouldn't this be vcpu->arch.virtual_tsc_khz?
> >
> > Yeah, I was thinking about that, but we need a vcpu instance for this.
>
> You can perhaps store the value from vcpu->arch.virtual_tsc_khz to
> kvm->arch when the MSR is first written?
>
> > Do you mean between HV_X64_MSR_REFERENCE_TSC which happens during
> > partition creation time and KVM_SET_CLOCK which happens during resume
> > after partition pause? If so - there are several differences, where
> > the offset calculation probably is the most important one.
>
> The offset and frequence are the only differences.
>
> + 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
So maybe hv_ref_time is not a good name, but we use it
for keeping the old vTSC value, saved before stopping VM.
Vadim.
>
> By the way, a small nit:
>
> >
> > + tsc_ref.tsc_sequence =
> > + boot_cpu_has(X86_FEATURE_CONSTANT_TSC) ? 1 : 0;
> > + tsc_ref.tsc_scale =
> > + ((10000LL << 32) / vcpu->arch.virtual_tsc_khz) << 32;
> > + tsc_ref.tsc_offset = 0;
> > if (__copy_to_user((void __user *)addr, &tsc_ref, sizeof(tsc_ref)))
> > return 1;
> > mark_page_dirty(kvm, gfn);
> > kvm->arch.hv_tsc_page = data;
> > + kvm->arch.hv_ref_count = 0;
> > break;
>
> This setting of kvm->arch.hv_ref_count belongs in the previous patch.
>
> Paolo
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