Re: [PATCH v12 3/3] KVM: s390: resetting the Topology-Change-Report

Pierre Morel
2022-07-12T07:20:39Z

On 7/11/22 15:22, Janis Schoetterl-Glausch wrote:
> On 7/11/22 10:41, Pierre Morel wrote:
>> During a subsystem reset the Topology-Change-Report is cleared.
>>
>> Let's give userland the possibility to clear the MTCR in the case
>> of a subsystem reset.
>>
>> To migrate the MTCR, we give userland the possibility to
>> query the MTCR state.
>>
>> We indicate KVM support for the CPU topology facility with a new
>> KVM capability: KVM_CAP_S390_CPU_TOPOLOGY.
>>
>> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
>
> Reviewed-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
>

Thanks!

> See nits/comments below.
>
>> ---
>> Documentation/virt/kvm/api.rst | 25 ++++++++++++++
>> arch/s390/include/uapi/asm/kvm.h | 1 +
>> arch/s390/kvm/kvm-s390.c | 56 ++++++++++++++++++++++++++++++++
>> include/uapi/linux/kvm.h | 1 +
>> 4 files changed, 83 insertions(+)
>>
>> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
>> index 11e00a46c610..5e086125d8ad 100644
>> --- a/Documentation/virt/kvm/api.rst
>> +++ b/Documentation/virt/kvm/api.rst
>> @@ -7956,6 +7956,31 @@ should adjust CPUID leaf 0xA to reflect that the PMU is disabled.
>> When enabled, KVM will exit to userspace with KVM_EXIT_SYSTEM_EVENT of
>> type KVM_SYSTEM_EVENT_SUSPEND to process the guest suspend request.
>>
>> +8.37 KVM_CAP_S390_CPU_TOPOLOGY
>> +------------------------------
>> +
>> +:Capability: KVM_CAP_S390_CPU_TOPOLOGY
>> +:Architectures: s390
>> +:Type: vm
>> +
>> +This capability indicates that KVM will provide the S390 CPU Topology
>> +facility which consist of the interpretation of the PTF instruction for
>> +the function code 2 along with interception and forwarding of both the
>> +PTF instruction with function codes 0 or 1 and the STSI(15,1,x)
>
> Is the architecture allowed to extend STSI without a facility?
> If so, if we say here that STSI 15.1.x is passed to user space, then
> I think we should have a
>
> if (sel1 != 1)
> goto out_no_data;
>
> or maybe even
>
> if (sel1 != 1 || sel2 < 2 || sel2 > 6)
> goto out_no_data;
>
> in priv.c

I am not a big fan of doing everything in the kernel.
Here we have no performance issue since it is an error of the guest if
it sends a wrong selector.

Even testing the facility or PV in the kernel is for my opinion arguable
in the case we do not do any treatment in the kernel.

I do not see what it brings to us, it increase the LOCs and makes the
implementation less easy to evolve.

>
>> +instruction to the userland hypervisor.
>> +
>> +The stfle facility 11, CPU Topology facility, should not be indicated
>> +to the guest without this capability.
>> +
>> +When this capability is present, KVM provides a new attribute group
>> +on vm fd, KVM_S390_VM_CPU_TOPOLOGY.
>> +This new attribute allows to get, set or clear the Modified Change
>
> get or set, now that there is no explicit clear anymore.

Yes now it is a set to 0 but the action of clearing remains.

>
>> +Topology Report (MTCR) bit of the SCA through the kvm_device_attr
>> +structure.> +
>> +When getting the Modified Change Topology Report value, the attr->addr
>
> When getting/setting the...
>
>> +must point to a byte where the value will be stored.
>
> ... will be stored/retrieved from.

OK

>> +
>> 9. Known KVM API problems
>> =========================
>>
>> diff --git a/arch/s390/include/uapi/asm/kvm.h b/arch/s390/include/uapi/asm/kvm.h
>> index 7a6b14874d65..a73cf01a1606 100644
>> --- a/arch/s390/include/uapi/asm/kvm.h
>> +++ b/arch/s390/include/uapi/asm/kvm.h
>> @@ -74,6 +74,7 @@ struct kvm_s390_io_adapter_req {
>> #define KVM_S390_VM_CRYPTO 2
>> #define KVM_S390_VM_CPU_MODEL 3
>> #define KVM_S390_VM_MIGRATION 4
>> +#define KVM_S390_VM_CPU_TOPOLOGY 5
>>
>> /* kvm attributes for mem_ctrl */
>> #define KVM_S390_VM_MEM_ENABLE_CMMA 0
>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>> index 70436bfff53a..b18e0b940b26 100644
>> --- a/arch/s390/kvm/kvm-s390.c
>> +++ b/arch/s390/kvm/kvm-s390.c
>> @@ -606,6 +606,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>> case KVM_CAP_S390_PROTECTED:
>> r = is_prot_virt_host();
>> break;
>> + case KVM_CAP_S390_CPU_TOPOLOGY:
>> + r = test_facility(11);
>> + break;
>> default:
>> r = 0;
>> }
>> @@ -817,6 +820,20 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
>> icpt_operexc_on_all_vcpus(kvm);
>> r = 0;
>> break;
>> + case KVM_CAP_S390_CPU_TOPOLOGY:
>> + r = -EINVAL;
>> + mutex_lock(&kvm->lock);
>> + if (kvm->created_vcpus) {
>> + r = -EBUSY;
>> + } else if (test_facility(11)) {
>> + set_kvm_facility(kvm->arch.model.fac_mask, 11);
>> + set_kvm_facility(kvm->arch.model.fac_list, 11);
>> + r = 0;
>> + }
>> + mutex_unlock(&kvm->lock);
>> + VM_EVENT(kvm, 3, "ENABLE: CAP_S390_CPU_TOPOLOGY %s",
>> + r ? "(not available)" : "(success)");
>> + break;
>> default:
>> r = -EINVAL;
>> break;
>> @@ -1717,6 +1734,36 @@ static void kvm_s390_update_topology_change_report(struct kvm *kvm, bool val)
>> read_unlock(&kvm->arch.sca_lock);
>> }
>>
>> +static int kvm_s390_set_topology(struct kvm *kvm, struct kvm_device_attr *attr)
>
> kvm_s390_set_topology_changed maybe?
> kvm_s390_get_topology_changed below then.

No strong opinion, if you prefer I change this.

>
>> +{
>> + if (!test_kvm_facility(kvm, 11))
>> + return -ENXIO;
>> +
>> + kvm_s390_update_topology_change_report(kvm, !!attr->attr);
>> + return 0;
>> +}
>> +
>> +static int kvm_s390_get_topology(struct kvm *kvm, struct kvm_device_attr *attr)
>> +{
>> + union sca_utility utility;
>> + struct bsca_block *sca;
>> + __u8 topo;
>> +
>> + if (!test_kvm_facility(kvm, 11))
>> + return -ENXIO;
>> +
>> + read_lock(&kvm->arch.sca_lock);
>> + sca = kvm->arch.sca;
>> + utility.val = READ_ONCE(sca->utility.val);
>
> I don't think you need the READ_ONCE anymore, now that there is a lock it should act as a compile barrier.

I think you are right.

>> + read_unlock(&kvm->arch.sca_lock);
>> + topo = utility.mtcr;
>> +
>> + if (copy_to_user((void __user *)attr->addr, &topo, sizeof(topo)))
>
> Why void not u8?

I like to say we write on "topo" with the size of "topo".
So we do not need to verify the effective size of topo.
But I understand, it is a UAPI, setting u8 in the copy_to_user makes
sense too.
For my personal opinion, I would have prefer that userland tell us the
size it awaits even here, for this special case, since we use a byte, we
can not do really wrong.

>
>> + return -EFAULT;
>> +
>> + return 0;
>> +}
>> +
> [...]
>

--
Pierre Morel
IBM Lab Boeblingen