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Date:   Mon, 27 Jul 2020 18:25:50 +0100
From:   Marc Zyngier <maz@kernel.org>
To:     Andrew Jones <drjones@redhat.com>
Cc:     kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu,
        pbonzini@redhat.com, steven.price@arm.com
Subject: Re: [PATCH 1/5] KVM: arm64: pvtime: steal-time is only supported when
 configured
In-Reply-To: <20200711100434.46660-2-drjones@redhat.com>
References: <20200711100434.46660-1-drjones@redhat.com>
 <20200711100434.46660-2-drjones@redhat.com>
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Hi Andrew,

On 2020-07-11 11:04, Andrew Jones wrote:
> Don't confuse the guest by saying steal-time is supported when
> it hasn't been configured by userspace and won't work.
> 
> Signed-off-by: Andrew Jones <drjones@redhat.com>
> ---
>  arch/arm64/kvm/pvtime.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/pvtime.c b/arch/arm64/kvm/pvtime.c
> index f7b52ce1557e..2b22214909be 100644
> --- a/arch/arm64/kvm/pvtime.c
> +++ b/arch/arm64/kvm/pvtime.c
> @@ -42,9 +42,12 @@ long kvm_hypercall_pv_features(struct kvm_vcpu 
> *vcpu)
> 
>  	switch (feature) {
>  	case ARM_SMCCC_HV_PV_TIME_FEATURES:
> -	case ARM_SMCCC_HV_PV_TIME_ST:
>  		val = SMCCC_RET_SUCCESS;
>  		break;
> +	case ARM_SMCCC_HV_PV_TIME_ST:
> +		if (vcpu->arch.steal.base != GPA_INVALID)
> +			val = SMCCC_RET_SUCCESS;
> +		break;
>  	}
> 
>  	return val;

I'm not so sure about this. I have always considered the
discovery interface to be "do you know about this SMCCC
function". And if you look at the spec, it says (4.2,
PV_TIME_FEATURES):

<quote>
If PV_call_id identifies PV_TIME_FEATURES, this call returns:
• NOT_SUPPORTED (-1) to indicate that all
paravirtualized time functions in this specification are not
supported.
• SUCCESS (0) to indicate that all the paravirtualized time
functions in this specification are supported.
</quote>

So the way I understand it, you cannot return "supported"
for PV_TIME_FEATURES, and yet return NOT_SUPPORTED for
PV_TIME_ST. It applies to *all* features.

Yes, this is very bizarre. But I don't think we can deviate
from it.

Thanks,

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