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

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Date:   Fri, 18 Feb 2022 19:24:33 +0100
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Janosch Frank <frankja@...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 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.

Regards,
Pierre

-- 
Pierre Morel
IBM Lab Boeblingen