Re: [RFC PATCH v11 5/9] psci: Add hypercall service for ptp_kvm.

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Apr 28, 2020 at 07:14:52AM +0100, Jianyong Wu wrote:
> On 2020/4/24 6:39 PM, Mark Rutland wrote:
> > On Fri, Apr 24, 2020 at 03:50:22AM +0100, Jianyong Wu wrote:
> >> On 2020/4/21 5:57 PM, Mark Rutland wrote:
> >>> On Tue, Apr 21, 2020 at 11:23:00AM +0800, Jianyong Wu wrote:
> >>>> diff --git a/virt/kvm/arm/hypercalls.c b/virt/kvm/arm/hypercalls.c
> >>>> index 550dfa3e53cd..a5309c28d4dc 100644
> >>>> --- a/virt/kvm/arm/hypercalls.c
> >>>> +++ b/virt/kvm/arm/hypercalls.c
> >>>> @@ -62,6 +66,44 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
> >>>>    if (gpa != GPA_INVALID)
> >>>>    val = gpa;
> >>>>    break;
> >>>> +/*
> >>>> + * This serves virtual kvm_ptp.
> >>>> + * Four values will be passed back.
> >>>> + * reg0 stores high 32-bit host ktime;
> >>>> + * reg1 stores low 32-bit host ktime;
> >>>> + * reg2 stores high 32-bit difference of host cycles and cntvoff;
> >>>> + * reg3 stores low 32-bit difference of host cycles and cntvoff.
> >>>> + */
> >>>> +case ARM_SMCCC_HYP_KVM_PTP_FUNC_ID:
> >>> Shouldn't the host opt-in to providing this to the guest, as with other
> >>> features?
> >> er, do you mean that "ARM_SMCCC_HV_PV_TIME_XXX" as "opt-in"? if so, I
> >> think this
> >>
> >> kvm_ptp doesn't need a buddy. the driver in guest will call this service
> >> in a definite way.
> > I mean that when creating the VM, userspace should be able to choose
> > whether the PTP service is provided to the guest. The host shouldn't
> > always provide it as there may be cases where doing so is undesireable.
> >
> I think I have implemented in patch 9/9 that userspace can get the info
> that if the host offers the kvm_ptp service. But for now, the host
> kernel will always offer the kvm_ptp capability in the current
> implementation. I think x86 follow the same behavior (see [1]). so I
> have not considered when and how to disable this kvm_ptp service in
> host. Do you think we should offer this opt-in?

I think taht should be opt-in, yes.

[...]

> > It's also not clear to me what notion of host time is being exposed to
> > the guest (and consequently how this would interact with time changes on
> > the host, time namespaces, etc). Having some description of that would
> > be very helpful.
> 
> sorry to have not made it clear.
> 
> Time will not change in host and only time in guest will change to sync
> with host. host time is the target that time in guest want to adjust to.
> guest need to get the host time then compute the different of the time
> in guest and host, so the guest can adjust the time base on the difference.

I understood that host time wouldn't change here, but what was not clear
is which notion of host time is being exposed to the guest.

e.g. is that a raw monotonic clock, or one subject to periodic adjument,
or wall time in the host? What is the epoch of the host time?

> I will add the base principle of time sync service in guest using
> kvm_ptp in commit message.

That would be great; thanks!

Mark.