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Date:   Mon, 07 Sep 2020 10:47:01 +0100
From:   Marc Zyngier <maz@kernel.org>
To:     Jianyong Wu <Jianyong.Wu@arm.com>
Cc:     netdev@vger.kernel.org, yangbo.lu@nxp.com, john.stultz@linaro.org,
        tglx@linutronix.de, pbonzini@redhat.com,
        sean.j.christopherson@intel.com, richardcochran@gmail.com,
        Mark Rutland <Mark.Rutland@arm.com>, will@kernel.org,
        Suzuki Poulose <Suzuki.Poulose@arm.com>,
        Steven Price <Steven.Price@arm.com>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
        Steve Capper <Steve.Capper@arm.com>,
        Justin He <Justin.He@arm.com>, nd <nd@arm.com>
Subject: Re: [PATCH v14 08/10] ptp: arm64: Enable ptp_kvm for arm64
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On 2020-09-07 10:28, Jianyong Wu wrote:
>> -----Original Message-----
>> From: Marc Zyngier <maz@kernel.org>
>> Sent: Monday, September 7, 2020 4:55 PM
>> To: Jianyong Wu <Jianyong.Wu@arm.com>

[...]

>> >> 	arm_smccc_1_1_invoke(ARM_SMCCC_VENDOR_HYP_KVM_FEATUR
>> >> ES_FUNC_ID,
>> >> > +			     &hvc_res);
>> >> > +	if (!(hvc_res.a0 | BIT(ARM_SMCCC_KVM_FUNC_KVM_PTP)))
>> >> > +		return -EOPNOTSUPP;
>> >> > +
>> >> > +	return 0;
>> >>
>> >> What happens if the
>> >> ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID function isn't
>> implemented
>> >> (on an old kernel or a non-KVM hypervisor)? The expected behaviour is
>> >> that a0 will contain SMCCC_RET_NOT_SUPPORTED, which is -1.
>> >> The result is that this function always returns "supported". Not an
>> >> acceptable behaviour.
>> >>
>> > Oh!  it's really a stupid mistake, should be "&" not "|".
>> 
>> But even then. (-1 & whatever) is always true.
> 
> Yeah, what about checking if a0 is non-negative first? Like:
> if (hvc_res.a0 < 0 || !(hvc_res.a0 & BIT(ARM_SMCCC_KVM_FUNC_KVM_PTP)))
> 	return -EOPNOTSUPP;

I don't get it. You already carry a patch from Will that gives
you a way to check for a service (kvm_arm_hyp_service_available()).

Why do you need to reinvent the wheel?

         M.
-- 
Jazz is not dead. It just smells funny...