linux-kernel - Re: [RFC PATCH v3 4/6] psci: Add hvc call service for ptp

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Date:   Tue, 24 Sep 2019 16:20:14 +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 23/09/19 06:57, Jianyong Wu (Arm Technology China) wrote:
>> On 19/09/19 11:46, Jianyong Wu (Arm Technology China) wrote:
>>>> On 18/09/19 11:57, Jianyong Wu (Arm Technology China) wrote:
>>>>> Paolo Bonzini wrote:
>>>>>> 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].
>>>
>>> Val[] should be long not u32 I think, so in arm64 I can avoid that
>>> Y2038_safe, but also need rewrite for arm32.
>>
>> I don't think there's anything inherently wrong with u32 val[], and as you
>> notice it lets you reuse code between arm and arm64.  It's up to you and
>> Marc to decide.
>>
> To compatible 32-bit, Integrates second value and nanosecond value as a nanosecond value then split it into val[0] and val[1] and split cycle value into val[2] and val[3],
>  In this way, time will overflow at Y2262.
> WDYT?

So if I understand correctly you'd multiply by 10^9 (or better shift by
30) the nanoseconds.

That works, but why not provide 5 output registers?  Alternatively, take
an address as input and write there.

Finally, on x86 we added an argument for the CLOCK_* that is being read
(currently only CLOCK_REALTIME, but having room for extensibility in the
API is always nice).

Paolo