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

Pierre Morel
2019-02-27T08:09:19Z

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.

> 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.

> 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.

>
>
>>
>>>
>>> Message ID <342ffd56-b73a-b1f4-004d-de2c4aeef729@linux.ibm.com>
>>> Message ID <e04f0c8b-2fd9-1846-334a-faa48e0e051e@linux.ibm.com>
>>>
>>> You previously stated:
>>>
>>>     "QEMU and KVM can both accept PQAP/AQIC even if the vfio_ap
>>> driver is
>>>      not loaded. However now that the guest officially get the PQAP/AQIC
>>>      instruction we need to handle the specification and operation
>>>      exceptions inside KVM _before_ testing and even calling the driver
>>>      hook.
>>>
>>>      I will make the changes in the next iteration."
>>
>> Still seems right to me, and is done is this patch.
>> Isn't it?
>
> I don't think it's a matter of right and wrong, it's a matter of what
> makes sense. IMHO, you want to make things easy if other PQAP functions
> are intercepted at some time. In my opinion, there should be a switch
> statement in the pqap hook code with a case statement for each PQAP
> function supported by the hook. To plug in a new PQAP function handler,
> it will be a simple matter of writing the handler function and calling
> it from the case statement, like this:
>
> static int handle_pqap(struct kvm_vcpu *vcpu)
> {
>     int ret;
>     uint8_t fc;
>
>     fc = vcpu->run->s.regs.gprs[0] >> 24;
>
>     switch (fc) {
>     case 0x03:
>         ret = handle_pqap_aqic(vcpu);
>     default:
>         ret = -EOPNOTSUPP;
>     }
>
>     return ret;
> }
>
> That function belongs in the pqap hook. I see no reaason whatsoever to
> check the function code here. If there is no hook, then you will fall
> through to the instruction below:
>
> status.response_code = 0x01;

See answer above, what you are speaking about is the execution of the
instruction, but there can be exceptions during the decode of the
instruction.

>
>>
>>>
>>> I don't know what any of the above has to do with checking FC=0x03? If
>>> that check is moved to the pqap handler hook, it can just as well return
>>> -EOPNOTSUPP. In fact, down below you do this:
>>>
>>>      return vcpu->kvm->arch.crypto.pqap_hook(vcpu);
>>>
>>> If the RC=0x03 check fails in the hook, it will return -EOPNOTSUPP just
>>> like above. None of this is critical, but the parsing of the register
>>> values for the PQAP(AQIC) function ought to be done in the code that
>>> handles the PQAP instruction IMHO.
>>
>>
>> This interception code must handle the PQAP/AQIC instruction when the
>> hook is not used and should not modify the handling for other PQAP
>> instructions.
>> We can not move anything inside the hook that must be always done.
>
> What you are saying here makes no sense. If the check for the function
> code is moved into the pqap hook and fc != 0x03, the result will be
> exactly the same; the hook will return -EOPNOTSUPP.

again please consider that the hook may not be initialized.

Regards,
Pierre

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