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

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Message-ID: <797af843-6ed4-349c-55bf-73a0dab1249b@redhat.com>
Date:   Wed, 9 Oct 2019 22:56:43 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     John Stultz <john.stultz@...aro.org>
Cc:     "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>,
        "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>,
        "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 18:05, John Stultz wrote:
> On Wed, Oct 9, 2019 at 2:13 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>> 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.
> 
> I've not really looked at the code closely in awhile, so I'm not sure
> my suggestions will be too useful.
> 
> My only instinct is maybe to not include the clocksource pointer in
> the system_time_snapshot, as I worry that structure will then be
> abused by the interface users.  If you're just wanting to make sure
> the clocksource is what you're expecting, would instead putting only
> the clocksource name in the structure suffice?

Well, it would suffice but it would be quite ugly to do a string
comparison later.

What kind of abuse are you thinking of?  We already have struct
system_counterval_t for a clocksource+cycles tuple, so it seemed obvious
to me to make system_time_snapshot a superset of it...  In fact,
system_time_snapshot's cycles member is even unused currently, so it
could even be easily replaced by a struct system_counterval_t, instead
of adding an extra field.

Paolo