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

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Date:   Wed, 9 Oct 2019 11:13:25 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Jianyong Wu (Arm Technology China)" <Jianyong.Wu@....com>,
        Marc Zyngier <maz@...nel.org>,
        "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>,
        "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 09/10/19 10:18, Jianyong Wu (Arm Technology China) wrote:
> Hi Paolo,
> 
>> -----Original Message-----
>> From: Paolo Bonzini <pbonzini@...hat.com>
>> Sent: Wednesday, October 9, 2019 2:36 PM
>> To: Jianyong Wu (Arm Technology China) <Jianyong.Wu@....com>; Marc
>> Zyngier <maz@...nel.org>; netdev@...r.kernel.org; yangbo.lu@....com;
>> john.stultz@...aro.org; tglx@...utronix.de; sean.j.christopherson@...el.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; 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
>> Subject: Re: [RFC PATCH v3 4/6] psci: Add hvc call service for ptp_kvm.
>>
>> On 09/10/19 07:21, Jianyong Wu (Arm Technology China) wrote:
>>> As ptp_kvm clock has fixed to arm arch system counter in patch set v4,
>>> we need check if the current clocksource is system counter when return
>>> clock cycle in host, so a helper needed to return the current
>>> clocksource. Could I add this helper in next patch set?
>>
>> You don't need a helper.  You need to return the ARM arch counter
>> clocksource in the struct system_counterval_t that you return.
>> get_device_system_crosststamp will then check that the clocksource
>> matches the active one.
>
> We must ensure both of the host and guest using the same clocksource.
> get_device_system_crosststamp will check the clocksource of guest and we also need check
> the clocksource in host, and struct type can't be transferred from host to guest using arm hypercall.
> now we lack of a mechanism to check the current clocksource. I think this will be useful if we add one.

Got it---yes, I think adding a struct clocksource to struct
system_time_snapshot would make sense.  Then the hypercall can just use
ktime_get_snapshot and fail if the clocksource is not the ARM arch counter.

John (Stultz), does that sound good to you?  The context is that
Jianyong would like to add a hypercall that returns a (cycles,
nanoseconds) pair to the guest.  On x86 we're relying on the vclock_mode
field that is already there for the vDSO, but being able to just use
ktime_get_snapshot would be much nicer.

Paolo