Subject: Re: [RFC PATCH v3 4/6] psci: Add hvc call service for ptp_kvm.

From: Paolo Bonzini <[email-masked]>
Date: Wed, 18 Sep 2019 12:23:47 +0200

On 18/09/19 11:57, Jianyong Wu (Arm Technology China) wrote:
> Hi Paolo,
> 
>> On 18/09/19 10:07, Jianyong Wu wrote:
>>> +	case ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID:
>>> +		getnstimeofday(ts);
>>
>> This is not Y2038-safe.  Please use ktime_get_real_ts64 instead, and split the
>> 64-bit seconds value between val[0] and val[1].
>>
> As far as I know, y2038-safe will only affect signed 32-bit integer,
> how does it affect 64-bit integer?
> And why split 64-bit number into two blocks is necessary?

val is an u32, not an u64.  (And val[0], where you store the seconds, is
best treated as signed, since val[0] == -1 is returned for
SMCCC_RET_NOT_SUPPORTED).

>> However, it seems to me that the new function is not needed and you can
>> just use ktime_get_snapshot.  You'll get the time in systime_snapshot->real
>> and the cycles value in systime_snapshot->cycles.
> 
> See patch 5/6, I need both counter cycle and clocksource, ktime_get_snapshot seems only offer cycles.

No, patch 5/6 only needs the current clock (ptp_sc.cycles is never
accessed).  So you could just use READ_ONCE(tk->tkr_mono.clock).

However, even then I don't think it is correct to use ptp_sc.cs blindly
in patch 5.  I think there is a misunderstanding on the meaning of
system_counterval.cs as passed to get_device_system_crosststamp.
system_counterval.cs is not the active clocksource; it's the clocksource
on which system_counterval.cycles is based.

Hypothetically, the clocksource could be one for which ptp_sc.cycles is
_not_ a cycle value.  If you set system_counterval.cs to the system
clocksource, get_device_system_crosststamp will return a bogus value.
So system_counterval.cs should be set to something like
&clocksource_counter (from drivers/clocksource/arm_arch_timer.c).
Perhaps the right place to define kvm_arch_ptp_get_clock_fn is in that file?

>>> +		get_current_counterval(&sc);
>>> +		val[0] = ts->tv_sec;
>>> +		val[1] = ts->tv_nsec;
>>> +		val[2] = sc.cycles;
>>> +		val[3] = 0;
>>> +		break;
>>
>> This should return a guest-cycles value.  If the cycles values always the same
>> between the host and the guest on ARM, then okay.  If not, you have to
>> apply whatever offset exists.
>>
> In my opinion, when use ptp_kvm as clock sources to sync time
> between host and guest, user should promise the guest and host has no
> clock offset.

What would be the adverse effect of having a fixed offset between guest
and host?  If there were one, you'd have to check that and fail the
hypercall if there is an offset.  But again, I think it's enough to
subtract vcpu_vtimer(vcpu)->cntvoff or something like that.

You also have to check here that the clocksource is based on the ARM
architectural timer.  Again, maybe you could place the implementation in
drivers/clocksource/arm_arch_timer.c, and make it return -ENODEV if the
active clocksource is not clocksource_counter.  Then KVM can look for
errors and return SMCCC_RET_NOT_SUPPORTED in that case.

Thanks,

Paolo

> So we can be sure that the cycle between guest and
> host should be keep consistent. But I need check it.
> I think host cycle should be returned to guest as we should promise
> we get clock and counter in the same time.

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