linux-kernel - Re: [PATCH v7 1/1] s390x: KVM: guest support for topology function

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Date:   Mon, 21 Feb 2022 09:10:29 +0100
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     Pierre Morel <pmorel@...ux.ibm.com>,
        Nico Boehr <nrb@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        borntraeger@...ibm.com, cohuck@...hat.com, david@...hat.com,
        thuth@...hat.com, imbrenda@...ux.ibm.com, hca@...ux.ibm.com,
        gor@...ux.ibm.com, wintera@...ux.ibm.com, seiden@...ux.ibm.com
Subject: Re: [PATCH v7 1/1] s390x: KVM: guest support for topology function

On 2/18/22 19:24, Pierre Morel wrote:
> 
> 
> On 2/18/22 18:27, Pierre Morel wrote:
>>
>>
>> On 2/18/22 15:28, Janosch Frank wrote:
>>> On 2/18/22 14:13, Pierre Morel wrote:
>>>>
>>>>
>>>> On 2/17/22 18:17, Nico Boehr wrote:
>>>>> On Thu, 2022-02-17 at 10:59 +0100, Pierre Morel wrote:
>>>>> [...]
>>>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>>>> index 2296b1ff1e02..af7ea8488fa2 100644
>>>>>> --- a/arch/s390/kvm/kvm-s390.c
>>>>>> +++ b/arch/s390/kvm/kvm-s390.c
>>>>> [...]
>>>
>>> Why is there no interface to clear the SCA_UTILITY_MTCR on a subsystem
>>> reset?
>>
>> Right, I had one in my first version based on interception but I forgot
>> to implement an equivalent for KVM as I modified the implementation for
>> interpretation.
>> I will add this.
>>
>>>
>>>
>>>>>> -void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>>>>>> +/**
>>>>>> + * kvm_s390_vcpu_set_mtcr
>>>>>> + * @vcp: the virtual CPU
>>>>>> + *
>>>>>> + * Is only relevant if the topology facility is present.
>>>>>> + *
>>>>>> + * Updates the Multiprocessor Topology-Change-Report to signal
>>>>>> + * the guest with a topology change.
>>>>>> + */
>>>>>> +static void kvm_s390_vcpu_set_mtcr(struct kvm_vcpu *vcpu)
>>>>>>     {
>>>>>> +       struct esca_block *esca = vcpu->kvm->arch.sca;
>>>>>
>>>>> utility is at the same offset for the bsca and the esca, still
>>>>> wondering whether it is a good idea to assume esca here...
>>>>
>>>> We can take bsca to be coherent with the include file where we define
>>>> ESCA_UTILITY_MTCR inside the bsca.
>>>> And we can rename the define to SCA_UTILITY_MTCR as it is common for
>>>> both BSCA and ESCA the (E) is too much.
>>>
>>> Yes and maybe add a comment that it's at the same offset for esca so
>>> there won't come up further questions in the future.
>>
>> OK
>>
>>>
>>>>
>>>>>
>>>>> [...]
>>>>>> diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h
>>>>>> index 098831e815e6..af04ffbfd587 100644
>>>>>> --- a/arch/s390/kvm/kvm-s390.h
>>>>>> +++ b/arch/s390/kvm/kvm-s390.h
>>>>>> @@ -503,4 +503,29 @@ void kvm_s390_vcpu_crypto_reset_all(struct kvm
>>>>>> *kvm);
>>>>>>      */
>>>>>>     extern unsigned int diag9c_forwarding_hz;
>>>>>> +#define S390_KVM_TOPOLOGY_NEW_CPU -1
>>>>>> +/**
>>>>>> + * kvm_s390_topology_changed
>>>>>> + * @vcpu: the virtual CPU
>>>>>> + *
>>>>>> + * If the topology facility is present, checks if the CPU toplogy
>>>>>> + * viewed by the guest changed due to load balancing or CPU hotplug.
>>>>>> + */
>>>>>> +static inline bool kvm_s390_topology_changed(struct kvm_vcpu *vcpu)
>>>>>> +{
>>>>>> +       if (!test_kvm_facility(vcpu->kvm, 11))
>>>>>> +               return false;
>>>>>> +
>>>>>> +       /* A new vCPU has been hotplugged */
>>>>>> +       if (vcpu->arch.prev_cpu == S390_KVM_TOPOLOGY_NEW_CPU)
>>>>>> +               return true;
>>>>>> +
>>>>>> +       /* The real CPU backing up the vCPU moved to another socket
>>>>>> */
>>>>>> +       if (topology_physical_package_id(vcpu->cpu) !=
>>>>>> +           topology_physical_package_id(vcpu->arch.prev_cpu))
>>>>>> +               return true;
>>>>>
>>>>> Why is it OK to look just at the physical package ID here? What if the
>>>>> vcpu for example moves to a different book, which has a core with the
>>>>> same physical package ID?
>>>
>>> I'll need to look up stsi 15* output to understand this.
>>> But the architecture states that any change to the stsi 15 output sets
>>> the change bit so I'd guess Nico is correct.
>>>
>>
>> Yes, Nico is correct, as I already answered, however it is not any
>> change of stsi(15) but a change of stsi(15.1.2) output which sets the
>> change bit.
> 
> hum, that is what the POP says but in fact you are right a change of
> topology that changes the output of any STSI(15) sets the topology
> change report bit as the output of STSI(15.1.2) would be changed too
> obviously.

In this case I was just being too lazy to look up the correct query code 
but I knew it started with fc 15. It was Friday after all :-)

> 
> Regards,
> Pierre
>