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SubjectRE: [RFC PATCH v9 7/8] ptp: arm64: Enable ptp_kvm for arm64
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Hi Marc,

> -----Original Message-----
> From: Marc Zyngier <maz@kernel.org>
> Sent: Monday, January 13, 2020 7:21 PM
> To: Jianyong Wu <Jianyong.Wu@arm.com>
> Cc: netdev@vger.kernel.org; yangbo.lu@nxp.com; john.stultz@linaro.org;
> tglx@linutronix.de; pbonzini@redhat.com; sean.j.christopherson@intel.com;
> richardcochran@gmail.com; Mark Rutland <Mark.Rutland@arm.com>;
> will@kernel.org; Suzuki Poulose <Suzuki.Poulose@arm.com>; Steven Price
> <Steven.Price@arm.com>; linux-kernel@vger.kernel.org; linux-arm-
> kernel@lists.infradead.org; kvmarm@lists.cs.columbia.edu;
> kvm@vger.kernel.org; Steve Capper <Steve.Capper@arm.com>; Kaly Xin
> <Kaly.Xin@arm.com>; Justin He <Justin.He@arm.com>; nd <nd@arm.com>
> Subject: Re: [RFC PATCH v9 7/8] ptp: arm64: Enable ptp_kvm for arm64
>
> On 2020-01-13 10:37, Jianyong Wu wrote:
> > Hi Marc,
> >
> >> -----Original Message-----
> >> From: Marc Zyngier <maz@kernel.org>
> >> Sent: Friday, January 10, 2020 6:35 PM
> >> To: Jianyong Wu <Jianyong.Wu@arm.com>
> >> Cc: netdev@vger.kernel.org; yangbo.lu@nxp.com;
> >> john.stultz@linaro.org; tglx@linutronix.de; pbonzini@redhat.com;
> >> sean.j.christopherson@intel.com; richardcochran@gmail.com; Mark
> >> Rutland <Mark.Rutland@arm.com>; will@kernel.org; Suzuki Poulose
> >> <Suzuki.Poulose@arm.com>; Steven Price <Steven.Price@arm.com>;
> >> linux-kernel@vger.kernel.org; linux-arm- kernel@lists.infradead.org;
> >> kvmarm@lists.cs.columbia.edu; kvm@vger.kernel.org; Steve Capper
> >> <Steve.Capper@arm.com>; Kaly Xin <Kaly.Xin@arm.com>; Justin He
> >> <Justin.He@arm.com>; nd <nd@arm.com>
> >> Subject: Re: [RFC PATCH v9 7/8] ptp: arm64: Enable ptp_kvm for arm64
> >>
> >> Hi Jianyong,
> >>
> >> On 2020-01-10 10:15, Jianyong Wu wrote:
> >> > Hi Marc,
> >>
> >> [...]
> >>
> >> >> >> > + ktime_overall = hvc_res.a0 << 32 | hvc_res.a1;
> >> >> >> > + *ts = ktime_to_timespec64(ktime_overall);
> >> >> >> > + *cycle = hvc_res.a2 << 32 | hvc_res.a3;
> >> >> >>
> >> >> >> So why isn't that just a read of the virtual counter, given
> >> >> >> that what you do in the hypervisor seems to be "cntpct - cntvoff"?
> >> >> >>
> >> >> >> What am I missing here?
> >> >> >>
> >> >> > We need get clock time and counter cycle at the same time, so we
> >> >> > can't just read virtual counter at guest and must get it from host.
> >> >>
> >> >> See my comment in my reply to patch #6: *Must* seems like a very
> >> >> strong word, and you don't explain *why* that's better than just
> >> >> computing the total hypercall cost. Hint: given the frequency of
> >> >> the counter (in the few MHz
> >> >> range) vs the frequency of a CPU (in the multiple GHz range, and
> >> >> with an IPC close enough to 1), I doubt that you'll see the
> >> >> counter making much progress across a hypercall.
> >> >>
> >> > Sorry, I will avoid to use those strong words.
> >> >
> >> > It's really the case that the hypercall won't across cycle in general.
> >> > But sometimes, kernel preempt
> >> > may happen in the middle of the hypercall which we can't assume how
> >> > long before schedule back. so it's better capture them together at
> >> > the same time.
> >>
> >> Fair enough. Please document the rational, as I guess others will ask
> >> the same questions.
> >>
> > Ok
> >
> >> Then the problem to solve is that of the reference counter, as you so
> >> far assume the virtual counter. I guess you need to be able to let
> >> the guest select the reference counter when calling the PTP service.
> >>
> > I could not come up with an idea about the point where the guest give
> > this info of counter value.
> > Where we give that interface to ptp service, as it's not a user space
> > application.
>
> Again: why don't you let the guest ask for the counter it wants as part of the
> SMC call? What is preventing this?
>
Ok, let me try it

Thanks

> M.
> --
> Jazz is not dead. It just smells funny...

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