linux-kernel - Re: [PATCH v10 11/26] s390: vfio-ap: implement mediated device open callback

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Date:   Mon, 24 Sep 2018 20:40:22 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>,
        Tony Krowiak <akrowiak@...ux.vnet.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     freude@...ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, borntraeger@...ibm.com,
        cohuck@...hat.com, kwankhede@...dia.com,
        bjsdjshi@...ux.vnet.ibm.com, pbonzini@...hat.com,
        alex.williamson@...hat.com, pmorel@...ux.vnet.ibm.com,
        alifm@...ux.vnet.ibm.com, mjrosato@...ux.vnet.ibm.com,
        jjherne@...ux.vnet.ibm.com, thuth@...hat.com,
        pasic@...ux.vnet.ibm.com, berrange@...hat.com,
        fiuczy@...ux.vnet.ibm.com, buendgen@...ibm.com,
        frankja@...ux.ibm.com
Subject: Re: [PATCH v10 11/26] s390: vfio-ap: implement mediated device open
 callback

On 24/09/2018 18:07, Tony Krowiak wrote:
> On 09/24/2018 04:40 AM, David Hildenbrand wrote:
>>
>>>    /**
>>> - * Verify that the AP instructions are available on the guest. This is
>>> indicated
>>> - * via the  KVM_S390_VM_CPU_FEAT_AP CPU model feature.
>>> + * Verify that the AP instructions are being interpreted by firmware
>>> for the
>>> + * guest. This is indicated by the kvm->arch.crypto.apie flag.
>>>     */
>>>    static int kvm_ap_validate_crypto_setup(struct kvm *kvm)
>>>    {
>>> -	if (test_bit_inv(KVM_S390_VM_CPU_FEAT_AP, kvm->arch.cpu_feat))
>>> +	if (kvm->arch.crypto.apie)
>>>    		return 0;
>>
>> I wonder if this check makes sense, because apie can be toggled during
>> runtime. I guess it would be sufficient to check if the ap control block
>> is available and apie is supported by the HW.
> 
> I am not clear about what you are getting at here, but I'll attempt
> to respond. There is no need to check if the AP control block (CRYCB)
> is available as the address is set in the CRYCBD three instructions
> above, even if AP instructions are not available. Regarding whether apie 
> is supported by the hardware, the value of vcpu->kvm->arch.crypto.apie 
> can not be set unless it is supported by the HW. In the patch (24/26) 
> that provides the VM attributes to toggle this value, it can only be 
> turned on if the AP instructions are available. I might also note that 
> the kvm_ap_validate_crypto_setup() function is called whenever one of 
> the VM crypto attributes is changed, so it makes sense that decisions 
> made in this function are based on a change to a VM crypto attribute. In 
> my first pass at changing this function, I checked
> ap_instructions_available() here, but after considering all of the
> above, it made sense to me to check the apie flag.
> 

I prefer ap_instructions_available(). As I said, kvm->arch.crypto.apie
is a moving target.

-- 

Thanks,

David / dhildenb