On 2020-01-13 10:37, Jianyong Wu wrote:
Hi Marc,
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From: Marc Zyngier <maz@xxxxxxxxxx>
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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...
