netdev - Re: [PATCH v14 08/10] ptp: arm64: Enable ptp

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Message-ID: <87363vmdh6.wl-maz@kernel.org>
Date:   Sun, 06 Sep 2020 11:00:37 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Jianyong Wu <jianyong.wu@....com>
Cc:     netdev@...r.kernel.org, yangbo.lu@....com, john.stultz@...aro.org,
        tglx@...utronix.de, pbonzini@...hat.com,
        sean.j.christopherson@...el.com, richardcochran@...il.com,
        Mark.Rutland@....com, will@...nel.org, suzuki.poulose@....com,
        steven.price@....com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, Steve.Capper@....com, justin.he@....com,
        nd@....com
Subject: Re: [PATCH v14 08/10] ptp: arm64: Enable ptp_kvm for arm64

On Sat, 05 Sep 2020 12:01:42 +0100,
Marc Zyngier <maz@...nel.org> wrote:
> 
> On Fri, 04 Sep 2020 10:27:42 +0100,
> Jianyong Wu <jianyong.wu@....com> wrote:

[...]

> > +{
> > +	ktime_t ktime;
> > +
> > +	arm_smccc_1_1_invoke(ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID,
> > +			     hvc_res);
> > +	if ((long long)(hvc_res->a0) < 0)
> > +		return -EOPNOTSUPP;
> 
> Really? What if the cycle counter is a full 64 bit value, as it is
> *mandated* on ARMv8.6? It means that the counter is now invalid for
> half the lifetime of the system. Not acceptable either.

Having re-read this, this field doesn't contain the cycle counter, but
the time in ns. So checking for a negative value should actually be
fine for quite a while. My other comments still stand though.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.