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Date:   Tue, 12 Jul 2022 09:24:52 +0200
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Subject: Re: [PATCH v12 3/3] KVM: s390: resetting the Topology-Change-Report
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To:     Janis Schoetterl-Glausch <scgl@linux.ibm.com>, kvm@vger.kernel.org
Cc:     linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        borntraeger@de.ibm.com, frankja@linux.ibm.com, cohuck@redhat.com,
        david@redhat.com, thuth@redhat.com, imbrenda@linux.ibm.com,
        hca@linux.ibm.com, gor@linux.ibm.com, wintera@linux.ibm.com,
        seiden@linux.ibm.com, nrb@linux.ibm.com
References: <20220711084148.25017-1-pmorel@linux.ibm.com>
 <20220711084148.25017-4-pmorel@linux.ibm.com>
 <58016efc-9053-b743-05d6-4ace4dcdc2a8@linux.ibm.com>
From:   Pierre Morel <pmorel@linux.ibm.com>
In-Reply-To: <58016efc-9053-b743-05d6-4ace4dcdc2a8@linux.ibm.com>
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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