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Message-ID: <629538ea-13fb-e666-8df6-8ad23f114755@redhat.com>
Date: Wed, 18 Sep 2019 12:23:47 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: "Jianyong Wu (Arm Technology China)" <Jianyong.Wu@....com>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
"yangbo.lu@....com" <yangbo.lu@....com>,
"john.stultz@...aro.org" <john.stultz@...aro.org>,
"tglx@...utronix.de" <tglx@...utronix.de>,
"sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
"maz@...nel.org" <maz@...nel.org>,
"richardcochran@...il.com" <richardcochran@...il.com>,
Mark Rutland <Mark.Rutland@....com>,
Will Deacon <Will.Deacon@....com>,
Suzuki Poulose <Suzuki.Poulose@....com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
Steve Capper <Steve.Capper@....com>,
"Kaly Xin (Arm Technology China)" <Kaly.Xin@....com>,
"Justin He (Arm Technology China)" <Justin.He@....com>,
nd <nd@....com>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH v3 4/6] psci: Add hvc call service for ptp_kvm.
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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