linux-kernel - Re: [PATCH v4 1/7] s390: ap: kvm: add PQAP interception for AQIC

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Message-Id: <dc483b20-8ade-2d5f-6a76-a2c5d2e72c57@linux.ibm.com>
Date:   Thu, 28 Feb 2019 14:10:32 +0100
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        Tony Krowiak <akrowiak@...ux.ibm.com>
Cc:     alex.williamson@...hat.com, cohuck@...hat.com,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, frankja@...ux.ibm.com, pasic@...ux.ibm.com,
        david@...hat.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, freude@...ux.ibm.com, mimu@...ux.ibm.com
Subject: Re: [PATCH v4 1/7] s390: ap: kvm: add PQAP interception for AQIC

On 28/02/2019 12:03, Christian Borntraeger wrote:
> 
> 
> On 28.02.2019 10:42, Christian Borntraeger wrote:
> [...]
>>> Okay, let's go back to the genesis of this discussion; namely, my
>>> suggestion about moving the fc == 0x03 check into the hook code. If
>>> the vfio_ap module is not loaded, there will be no hook code. In that
>>> case, the check for the hook will fail and ultimately response code
>>> 0x01 will be set in the status word (which may not be the right thing
>>> to do?). You have not stated a single good reason for keeping this
>>> check, but I'm done with this silly argument. It certainly doesn't
>>> hurt anything.
>>
>> The instruction handler must handle the basic checks for the
>> instruction itself as outlined above.
>>
>> Do we want to allow QEMU to fully emulate everything (the  ECA_APIE case being off)?
>> The we should pass along everything to QEMU, but this is already done with the
>> ECA_APIE check, correct?
>>
>> Do we agree that when we are beyond the ECA_APIE check, that we do not emulate
>> in QEMU and we have enabled the AP instructions interpretion?
>> If yes then this has some implication:
>>
>> 1. ECA is on and we should only get PQAP interception for specific FC (namely 3).
>> 2. What we certainly should check is the facility bit of the guest (65) and reject fc==3
>> right away with a specification exception. I do not want the hook to mess with
>> the kvm cpu model. @Pierre would be good to actually check test_kvm_facility(vcpu->kvm, 65))
>> 3. What shall we do when fc == 0x3? We can certainly do the check here OR in the
>> hook. As long as we have only fc==3 this does not matter.
>>
>> Correct?
> 
> Thinking more about that, I think we should inject a specification exception for all
> unknown FCc != 0x3. That would also qualify for keeping it in the instruction handler.
> 

May be return a privileged operation exception if issued from guest's 
program state, but generally I agree with the idea of handling all PQAP 
functions here.

Regards,
Pierre

-- 
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany