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

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Message-ID: <HE1PR0801MB1676115C248E6DF09F9DD5A6F4950@HE1PR0801MB1676.eurprd08.prod.outlook.com>
Date:   Wed, 9 Oct 2019 05:21:13 +0000
From:   "Jianyong Wu (Arm Technology China)" <Jianyong.Wu@....com>
To:     Paolo Bonzini <pbonzini@...hat.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.

Hi Paolo,

> -----Original Message-----
> From: Paolo Bonzini <pbonzini@...hat.com>
> Sent: Thursday, September 19, 2019 8:13 PM
> To: Marc Zyngier <maz@...nel.org>; Jianyong Wu (Arm Technology China)
> <Jianyong.Wu@....com>; 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 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.
> 
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?

Thanks
Jianyong wu

> 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.