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

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Message-ID: <56a5b885-62c8-c4ef-e2f8-e945c0eb700e@redhat.com>
Date:   Thu, 19 Sep 2019 14:13:13 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marc Zyngier <maz@...nel.org>,
        "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>,
        "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 19/09/19 13:39, Marc Zyngier wrote:
>> I don't think it's ugly but more important, using tk->tkr_mono.clock is
>> incorrect.  See how the x86 code hardcodes &kvm_clock, it's the same for
>> ARM.
> Not really. The guest kernel is free to use any clocksource it wishes.

Understood, in fact it's the same on x86.

However, for PTP to work, the cycles value returned by the clocksource
must match the one returned by the hypercall.  So for ARM
get_device_system_crosststamp must receive the arch timer clocksource,
so that it will return -ENODEV if the active clocksource is anything else.

Paolo

> In some cases, it is actually desirable (like these broken systems that
> cannot use an in-kernel irqchip...). Maybe it is that on x86 the guest
> only uses the kvm_clock, but that's a much harder sell on ARM. The fact
> that ptp_kvm assumes that the clocksource is fixed doesn't seem correct
> in that case.