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

Halil Pasic
2019-02-28T14:07:37Z

On Thu, 28 Feb 2019 14:47:35 +0100
Pierre Morel <pmorel@linux.ibm.com> wrote:

> 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).

Sorry what did Christian propose? I've lost you. Christian's initial
analysis assumed AFAIU that we only have or care for fc == 3.

BTW have you seen my response to Christians analysis and the changes I
proposed?

Regards,
Halil

> With this, I will have to split the checks to the right place.
>
> Thanks for the comments.
>
> Regards,
> Pierre
>
>