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

Pierre Morel
2019-02-28T13:47:35Z

On 28/02/2019 14:44, Christian Borntraeger wrote:
>
>
> On 28.02.2019 14:23, Pierre Morel wrote:
>> On 28/02/2019 10:42, Christian Borntraeger wrote:
>>>
>>>
>>> On 27.02.2019 19:00, Tony Krowiak wrote:
>>>> On 2/27/19 3:09 AM, Pierre Morel wrote:
>>>>> On 26/02/2019 16:47, Tony Krowiak wrote:
>>>>>> On 2/26/19 6:47 AM, Pierre Morel wrote:
>>>>>>> On 25/02/2019 19:36, Tony Krowiak wrote:
>>>>>>>> On 2/22/19 10:29 AM, Pierre Morel wrote:
>>>>>>>>> We prepare the interception of the PQAP/AQIC instruction for
>>>>>>>>> the case the AQIC facility is enabled in the guest.
>>>>>>>>>
>>>>>>>>> We add a callback inside the KVM arch structure for s390 for
>>>>>>>>> a VFIO driver to handle a specific response to the PQAP
>>>>>>>>> instruction with the AQIC command.
>>>>>>>>>
>>>>>>>>> We inject the correct exceptions from inside KVM for the case the
>>>>>>>>> callback is not initialized, which happens when the vfio_ap driver
>>>>>>>>> is not loaded.
>>>>>>>>>
>>>>>>>>> If the callback has been setup we call it.
>>>>>>>>> If not we setup an answer considering that no queue is available
>>>>>>>>> for the guest when no callback has been setup.
>>>>>>>>>
>>>>>>>>> We do consider the responsability of the driver to always initialize
>>>>>>>>> the PQAP callback if it defines queues by initializing the CRYCB for
>>>>>>>>> a guest.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
>>>>>>>
>>>>>>> ...snip...
>>>>>>>
>>>>>>>>> @@ -592,6 +593,55 @@ static int handle_io_inst(struct kvm_vcpu *vcpu)
>>>>>>>>>        }
>>>>>>>>>    }
>>>>>>>>> +/*
>>>>>>>>> + * handle_pqap: Handling pqap interception
>>>>>>>>> + * @vcpu: the vcpu having issue the pqap instruction
>>>>>>>>> + *
>>>>>>>>> + * We now support PQAP/AQIC instructions and we need to correctly
>>>>>>>>> + * answer the guest even if no dedicated driver's hook is available.
>>>>>>>>> + *
>>>>>>>>> + * The intercepting code calls a dedicated callback for this instruction
>>>>>>>>> + * if a driver did register one in the CRYPTO satellite of the
>>>>>>>>> + * SIE block.
>>>>>>>>> + *
>>>>>>>>> + * For PQAP/AQIC instructions only, verify privilege and specifications.
>>>>>>>>> + *
>>>>>>>>> + * If no callback available, the queues are not available, return this to
>>>>>>>>> + * the caller.
>>>>>>>>> + * Else return the value returned by the callback.
>>>>>>>>> + */
>>>>>>>>> +static int handle_pqap(struct kvm_vcpu *vcpu)
>>>>>>>>> +{
>>>>>>>>> +    uint8_t fc;
>>>>>>>>> +    struct ap_queue_status status = {};
>>>>>>>>> +
>>>>>>>>> +    /* Verify that the AP instruction are available */
>>>>>>>>> +    if (!ap_instructions_available())
>>>>>>>>> +        return -EOPNOTSUPP;
>>>>>>>>
>>>>>>>> How can the guest even execute an AP instruction if the AP instructions
>>>>>>>> are not available? If the AP instructions are not available on the host,
>>>>>>>> they will not be available on the guest (i.e., CPU model feature
>>>>>>>> S390_FEAT_AP will not be set). I suppose it doesn't hurt to check this
>>>>>>>> here given QEMU may not be the only client.
>>>>>>>>
>>>>>>>>> +    /* Verify that the guest is allowed to use AP instructions */
>>>>>>>>> +    if (!(vcpu->arch.sie_block->eca & ECA_APIE))
>>>>>>>>> +        return -EOPNOTSUPP;
>>>>>>>>> +    /* Verify that the function code is AQIC */
>>>>>>>>> +    fc = vcpu->run->s.regs.gprs[0] >> 24;
>>>>>>>>> +    if (fc != 0x03)
>>>>>>>>> +        return -EOPNOTSUPP;
>>>>>>>>
>>>>>>>> You must have missed my suggestion to move this to the
>>>>>>>> vcpu->kvm->arch.crypto.pqap_hook(vcpu) in the following responses:
>>>>>>>
>>>>>>> Please consider what happen if the vfio_ap module is not loaded.
>>>>>>
>>>>>> I have considered it and even verified my expectations empirically. If
>>>>>> the vfio_ap module is not loaded, you will not be able to create an mdev device.
>>>>>
>>>>> OK, now please consider that another userland tool, not QEMU uses KVM.
>>>>
>>>> What does that have to do with loading the vfio_ap module? Without the
>>>> vfio_ap module, there will be no AP devices for the guest. What are you
>>>> suggesting here?
>>>>
>>>>>
>>>>>> If you don't have an mdev device, you will not be able to
>>>>>> start a guest with a vfio-ap device. If you start a guest without a
>>>>>> vfio-ap device, but enable AP instructions for the guest, there will be
>>>>>> no AP devices attached to the guest. Without any AP devices attached,
>>>>>> the PQAP(AQIC) instructions will not ever get executed.
>>>>>
>>>>> This is not right. The instruction will be executed, eventually, after decoding.
>>>>
>>>> Please explain why the PQAP(AQIC) instruction will be executed on a
>>>> guest without any devices? Point me to the code in the AP bus where
>>>> PQAP(AQIC) is executed without a queue?
>>>
>>> The host must be prepared to handle malicous and broken guests. So if
>>> a guest does PQAP, we must handle that gracefully (e.g. by injecting an
>>> exception)
>>>
>>>>
>>>>>
>>>>>> Even if for some
>>>>>> unknown reason the PQAP(AQIC) instruction is executed - for some unknown
>>>>>> reason, it will fail with response code 0x01, AP-queue number not valid.
>>>>>
>>>>> No, before accessing the AP-queue the instruction will be decoded and depending on the installed micro-code it will fail with
>>>>> - OPERATION EXCEPTION if the micro-code is not installed
>>>>> - PRIVILEDGE OPERATION if the instruction is issued from userland (programm state)
>>>>> - SPECIFICATION exception if the instruction do not respect the usage specification
>>>>>
>>>>> then it will be interpreted by the microcode and access the queue and only then it will fail with RC 0x01, AP queue not valid.
>>>>>
>>>>> In the case of KVM, we intercept the instruction because it is issued by the guest and we set the AQIC facility on to force interception.
>>>>>
>>>>> KVM do for us all the decode steps I mention here above, if there is or not a pqap hook to be call to simulate the QP queue access.
>>>>>
>>>>> That done, the AP queue virtualisation can be called, this is done by calling the hook.
>>>>
>>>> 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))
>>
>>
>> Currently the check test_kvm_facility(vcpu->kvm, 65) is done in the instruction handler, what do you mean here?
>
> Found it. I think we should couple the check for 64 to fc==3. Otherwise both things are somewhat
> disconnected when reviewing.
>

Right.
In the next version I will go the way you proposed anyway and handle all
PQAP functions separatly (switch/dedicated functions).
With this, I will have to split the checks to the right place.

Thanks for the comments.

Regards,
Pierre

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