Subject: Re: [PATCH v14 08/10] ptp: arm64: Enable ptp_kvm for arm64

From: Marc Zyngier <[email-masked]>
Date: Mon, 07 Sep 2020 10:47:01 +0100

On 2020-09-07 10:28, Jianyong Wu wrote:
>> -----Original Message-----
>> From: Marc Zyngier <[email-masked]>
>> Sent: Monday, September 7, 2020 4:55 PM
>> To: Jianyong Wu <[email-masked]>

[...]

>> >> 	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...