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

[Jianyong Wu <Jianyong.Wu@arm.com>]

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;
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> 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.

> [...]
> 
> > By the way, does nested virtualization diff between arm64 and arm32?
> 
> There is no nested virt for 32bit (it is explicitly forbidden by the architecture).
>  
Ok, thanks

Thanks
Jianyong 

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