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

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Date:   Mon, 24 Sep 2018 14:43:43 -0400
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.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 09/24/2018 02:40 PM, David Hildenbrand wrote:
> 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.

Okay then.

>