Re: [PATCH v4 05/15] KVM: s390: enable/disable AP interpretive execution

Tony Krowiak
2018-04-17T16:34:16Z

On 04/17/2018 12:13 PM, Pierre Morel wrote:
> On 17/04/2018 17:02, Tony Krowiak wrote:
>> On 04/16/2018 06:51 AM, Pierre Morel wrote:
>>> On 15/04/2018 23:22, Tony Krowiak wrote:
>>>> The VFIO AP device model exploits interpretive execution of AP
>>>> instructions (APIE) to provide guests passthrough access to AP
>>>> devices. This patch introduces a new interface to enable and
>>>> disable APIE.
>>>>
>>>> Signed-off-by: Tony Krowiak <akrowiak@linux.vnet.ibm.com>
>>>> ---
>>>> arch/s390/include/asm/kvm-ap.h | 16 ++++++++++++++++
>>>> arch/s390/include/asm/kvm_host.h | 1 +
>>>> arch/s390/kvm/kvm-ap.c | 20 ++++++++++++++++++++
>>>> arch/s390/kvm/kvm-s390.c | 9 +++++++++
>>>> 4 files changed, 46 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/arch/s390/include/asm/kvm-ap.h
>>>> b/arch/s390/include/asm/kvm-ap.h
>>>> index 736e93e..a6c092e 100644
>>>> --- a/arch/s390/include/asm/kvm-ap.h
>>>> +++ b/arch/s390/include/asm/kvm-ap.h
>>>> @@ -35,4 +35,20 @@
>>>> */
>>>> void kvm_ap_build_crycbd(struct kvm *kvm);
>>>>
>>>> +/**
>>>> + * kvm_ap_interpret_instructions
>>>> + *
>>>> + * Indicate whether AP instructions shall be interpreted. If they
>>>> are not
>>>> + * interpreted, all AP instructions will be intercepted and routed
>>>> back to
>>>> + * userspace.
>>>> + *
>>>> + * @kvm: the virtual machine attributes
>>>> + * @enable: indicates whether AP instructions are to be
>>>> interpreted (true) or
>>>> + * or not (false).
>>>> + *
>>>> + * Returns 0 if completed successfully; otherwise, returns
>>>> -EOPNOTSUPP
>>>> + * indicating that AP instructions are not installed on the guest.
>>>> + */
>>>> +int kvm_ap_interpret_instructions(struct kvm *kvm, bool enable);
>>>> +
>>>> #endif /* _ASM_KVM_AP */
>>>> diff --git a/arch/s390/include/asm/kvm_host.h
>>>> b/arch/s390/include/asm/kvm_host.h
>>>> index 3162783..5470685 100644
>>>> --- a/arch/s390/include/asm/kvm_host.h
>>>> +++ b/arch/s390/include/asm/kvm_host.h
>>>> @@ -715,6 +715,7 @@ struct kvm_s390_crypto {
>>>> __u32 crycbd;
>>>> __u8 aes_kw;
>>>> __u8 dea_kw;
>>>> + __u8 apie;
>>>> };
>>>>
>>>> #define APCB0_MASK_SIZE 1
>>>> diff --git a/arch/s390/kvm/kvm-ap.c b/arch/s390/kvm/kvm-ap.c
>>>> index 991bae4..55d11b5 100644
>>>> --- a/arch/s390/kvm/kvm-ap.c
>>>> +++ b/arch/s390/kvm/kvm-ap.c
>>>> @@ -58,3 +58,23 @@ void kvm_ap_build_crycbd(struct kvm *kvm)
>>>> }
>>>> }
>>>> EXPORT_SYMBOL(kvm_ap_build_crycbd);
>>>> +
>>>> +int kvm_ap_interpret_instructions(struct kvm *kvm, bool enable)
>>>> +{
>>>> + int ret = 0;
>>>> +
>>>> + mutex_lock(&kvm->lock);
>>>> +
>>>> + if (!test_kvm_cpu_feat(kvm, KVM_S390_VM_CPU_FEAT_AP)) {
>>>
>>> Do we really need to test CPU_FEAT_AP?
>>
>> Yes we do.
>
> really? why?

Answered this in Message ID:
<2b053349-071e-17ed-6ebd-a37bcfd2f330@linux.vnet.ibm.com>

>
>>
>>>
>>>
>>> I understand that KVM_S390_VM_CPU_FEAT_AP means AP instructions are
>>> interpreted.
>>> shouldn't we add this information in the name?
>>> like KVM_S390_VM_CPU_FEAT_APIE
>>
>> KVM_S390_VM_CPU_FEAT_AP does NOT mean AP instructions are
>> interpreted, it means
>> AP instructions are installed.
>
> Right same error I made all along this review.
> But AFAIK it means AP instructions are provided to the guest.
> Then should this function be called if the guest has no AP instructions ?

Same answer as below. We have no control over who calls this interface, so
it behooves us to make sure it isn't called erroneously. I despise reading
code where I have to search all of the callers to ensure they perform a
required check ... why not just do it in the interface.

>
>
>
>>
>>>
>>>> + ret = -EOPNOTSUPP;
>>>> + goto done;
>>>> + }
>>>> +
>>>> + kvm->arch.crypto.apie = enable;
>>>> + kvm_s390_vcpu_crypto_reset_all(kvm);
>>>> +
>>>> +done:
>>>> + mutex_unlock(&kvm->lock);
>>>> + return ret;
>>>> +}
>>>> +EXPORT_SYMBOL(kvm_ap_interpret_instructions);
>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>> index 55cd897..1dc8566 100644
>>>> --- a/arch/s390/kvm/kvm-s390.c
>>>> +++ b/arch/s390/kvm/kvm-s390.c
>>>> @@ -1901,6 +1901,9 @@ static void kvm_s390_crypto_init(struct kvm
>>>> *kvm)
>>>> kvm->arch.crypto.crycb = &kvm->arch.sie_page2->crycb;
>>>> kvm_ap_build_crycbd(kvm);
>>>>
>>>> + /* Default setting indicating SIE shall interpret AP
>>>> instructions */
>>>> + kvm->arch.crypto.apie = 1;
>>>> +
>>>> if (!test_kvm_facility(kvm, 76))
>>>> return;
>>>>
>>>> @@ -2434,6 +2437,12 @@ static void
>>>> kvm_s390_vcpu_crypto_setup(struct kvm_vcpu *vcpu)
>>>> {
>>>> vcpu->arch.sie_block->crycbd = vcpu->kvm->arch.crypto.crycbd;
>>>>
>>>> + vcpu->arch.sie_block->eca &= ~ECA_APIE;
>>>> + if (vcpu->kvm->arch.crypto.apie &&
>>>> + test_kvm_cpu_feat(vcpu->kvm, KVM_S390_VM_CPU_FEAT_AP))
>>>
>>> Do we call xxx_crypto_setup() if KVM does not support AP
>>> interpretation?
>>
>> Yes, kvm_s390_vcpu_crypto_setup(vcpu) is called by
>> kvm_arch_vcpu_setup(vcpu)
>> as well as from kvm_s390_vcpu_crypto_reset_all(kvm). Calling it has
>> nothing
>> to do with whether AP interpretation is supported or not as it does much
>> more than that, including setting up of wrapping keys and the CRYCBD.
>
> Sorry, still the same error I made about CPU_FEAT_AP meaning AP
> instructions in the guest
> and not AP interpretation available.
> Could apie be set if AP instruction are not supported?

Only if code authors and reviewers ensure that no future code changes
set the apie flag
when the CPU_FEAT_AP is not set. Why do you see this as a problem? I see
it as
defensive coding since we have no control over who calls this interface.

>
>
>>
>>>
>>>> + vcpu->arch.sie_block->eca |= ECA_APIE;
>>>> +
>>>> +
>>>> if (!test_kvm_facility(vcpu->kvm, 76))
>>>> return;
>>>>
>>>
>>
>