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

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Date:   Fri, 18 Feb 2022 14:13:34 +0100
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Nico Boehr <nrb@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        borntraeger@...ibm.com, frankja@...ux.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 2/17/22 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
> [...]
>>   
>> -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.

> 
> [...]
>> 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?
> 

You are right, we should look at the drawer and book id too.
Something like that I think:

         if ((topology_physical_package_id(vcpu->cpu) !=
              topology_physical_package_id(vcpu->arch.prev_cpu)) ||
             (topology_book_id(vcpu->cpu) !=
              topology_book_id(vcpu->arch.prev_cpu)) ||
             (topology_drawer_id(vcpu->cpu) !=
              topology_drawer_id(vcpu->arch.prev_cpu)))
                 return true;


Thanks,
regards,
Pierre
-- 
Pierre Morel
IBM Lab Boeblingen