Messages in this thread | | | Subject | Re: [PATCH v3 04/14] KVM: s390: device attribute to set AP interpretive execution | From | Tony Krowiak <> | Date | Thu, 15 Mar 2018 13:21:54 -0400 |
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On 03/15/2018 11:45 AM, Pierre Morel wrote: > On 15/03/2018 16:26, Tony Krowiak wrote: >> On 03/15/2018 09:00 AM, Pierre Morel wrote: >>> On 14/03/2018 22:57, Halil Pasic wrote: >>>> >>>> On 03/14/2018 07:25 PM, 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 device attribute in the >>>>> KVM_S390_VM_CRYPTO device attribute group to set APIE from >>>>> the VFIO AP device defined on the guest. >>>>> >>>>> Signed-off-by: Tony Krowiak <akrowiak@linux.vnet.ibm.com> >>>>> --- >>>> [..] >>>> >>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c >>>>> index a60c45b..bc46b67 100644 >>>>> --- a/arch/s390/kvm/kvm-s390.c >>>>> +++ b/arch/s390/kvm/kvm-s390.c >>>>> @@ -815,6 +815,19 @@ static int kvm_s390_vm_set_crypto(struct kvm >>>>> *kvm, struct kvm_device_attr *attr) >>>>> sizeof(kvm->arch.crypto.crycb->dea_wrapping_key_mask)); >>>>> VM_EVENT(kvm, 3, "%s", "DISABLE: DEA keywrapping support"); >>>>> break; >>>>> + case KVM_S390_VM_CRYPTO_INTERPRET_AP: >>>>> + if (attr->addr) { >>>>> + if (!test_kvm_cpu_feat(kvm, KVM_S390_VM_CPU_FEAT_AP)) >>>> Unlock mutex before returning? >>>> >>>> Maybe flip conditions (don't allow manipulating apie if feature not >>>> there). >>>> Clearing the anyways clear apie if feature not there ain't too bad, >>>> but >>>> rejecting the operation appears nicer to me. >>>> >>>>> + return -EOPNOTSUPP; >>>>> + kvm->arch.crypto.apie = 1; >>>>> + VM_EVENT(kvm, 3, "%s", >>>>> + "ENABLE: AP interpretive execution"); >>>>> + } else { >>>>> + kvm->arch.crypto.apie = 0; >>>>> + VM_EVENT(kvm, 3, "%s", >>>>> + "DISABLE: AP interpretive execution"); >>>>> + } >>>>> + break; >>>>> default: >>>>> mutex_unlock(&kvm->lock); >>>>> return -ENXIO; >>>> I wonder how the loop after this switch works for >>>> KVM_S390_VM_CRYPTO_INTERPRET_AP: >>>> >>>> kvm_for_each_vcpu(i, vcpu, kvm) { >>>> kvm_s390_vcpu_crypto_setup(vcpu); >>>> exit_sie(vcpu); >>>> } >>>> >>>> From not doing something like for KVM_S390_VM_CRYPTO_INTERPRET_AP >>>> >>>> if (kvm->created_vcpus) { >>>> mutex_unlock(&kvm->lock); >>>> return -EBUSY; >>>> and from the aforementioned loop I guess ECA.28 can be changed >>>> for a running guest. >>>> >>>> If there are running vcpus when KVM_S390_VM_CRYPTO_INTERPRET_AP is >>>> changed (set) these will be taken out of SIE by exit_sie(). Then >>>> for the >>>> corresponding threads the control probably goes to QEMU (the >>>> emulator in >>>> the userspace). And it puts that vcpu back into the SIE, and then that >>>> cpu starts acting according to the new ECA.28 value. While other >>>> vcpus >>>> may still work with the old value of ECA.28. >>>> >>>> I'm not saying what I describe above is necessarily something broken. >>>> But I would like to have it explained, why is it OK -- provided I >>>> did not >>>> make any errors in my reasoning (assumptions included). >>>> >>>> Can you help me understand this code? >>>> >>>> Regards, >>>> Halil >>>> >>>> [..] >>>> >>> >>> I have the same concerns as Halil. >>> >>> We do not need to change the virtulization type >>> (hardware/software) on the fly for the current use case. >>> >>> Couldn't we delay this until we have one and in between only make >>> the vCPU hotplug clean? >>> >>> We only need to let the door open for the day we have such a use case. >> Are you suggesting this code be removed? If so, then where and under >> what conditions would >> you suggest setting ECA.28 given you objected to setting it based on >> whether the >> AP feature is installed? > > I would only call kvm_s390_vcpu_crypto_setup() from inside > kvm_arch_vcpu_init() > as it is already. It is not called from kvm_arch_vcpu_init(), it is called from kvm_arch_vcpu_setup(). Also, this loop was already here, I did not put it in. Assuming whomever put it there did so for a reason, it is not my place to remove it. According to a trace I ran, the calls to this function occur after the vcpus are created. Consequently, the kvm_s390_vcpu_crypto_setup() function would not be called without the loop and neither the key wrapping support nor the ECA_APIE would be configured in the vcpu's SIE descriptor.
If you have a better idea for where/how to set this flag, I'm all ears. It would be nice if it could be set before the vcpus are created, but I haven't found a good candidate. I suspect that the loop was put in to make sure that all vcpus get updated regardless of whether they are running or not, but I don't know what happens after a vcpu is kicked out of SIE. I suspect, as Halil surmised, that QEMU restores the vcpus to SIE. This would seemingly cause the kvm_arch_vcpu_setup() to get called at which time the ECA_APIE value as well as the key wrapping values will get set. If somebody has knowledge of the flow here, please feel free to pitch in. > > > >>> >>> >>> Pierre >>> >>> >>> >> >
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