Re: [RFC PATCH v9 7/8] ptp: arm64: Enable ptp_kvm for arm64

[Marc Zyngier]

On 2020-01-13 10:37, Jianyong Wu wrote:
> Hi Marc,
>
> > -----Original Message-----
> > From: Marc Zyngier <maz@xxxxxxxxxx>
> > Sent: Friday, January 10, 2020 6:35 PM
> > To: Jianyong Wu <Jianyong.Wu@xxxxxxx>
> > Cc: netdev@xxxxxxxxxxxxxxx; yangbo.lu@xxxxxxx; john.stultz@xxxxxxxxxx;
> > tglx@xxxxxxxxxxxxx; pbonzini@xxxxxxxxxx; 
> > sean.j.christopherson@xxxxxxxxx;
> > richardcochran@xxxxxxxxx; Mark Rutland <Mark.Rutland@xxxxxxx>;
> > will@xxxxxxxxxx; Suzuki Poulose <Suzuki.Poulose@xxxxxxx>; Steven Price
> > <Steven.Price@xxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; linux-arm-
> > kernel@xxxxxxxxxxxxxxxxxxx; kvmarm@xxxxxxxxxxxxxxxxxxxxx;
> > kvm@xxxxxxxxxxxxxxx; Steve Capper <Steve.Capper@xxxxxxx>; Kaly Xin
> > <Kaly.Xin@xxxxxxx>; Justin He <Justin.He@xxxxxxx>; nd <nd@xxxxxxx>
> > 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?

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