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Date:   Thu, 12 May 2022 11:52:50 +0200
From:   Claudio Imbrenda <imbrenda@linux.ibm.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Pierre Morel <pmorel@linux.ibm.com>, kvm@vger.kernel.org,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        borntraeger@de.ibm.com, frankja@linux.ibm.com, cohuck@redhat.com,
        thuth@redhat.com, hca@linux.ibm.com, gor@linux.ibm.com,
        wintera@linux.ibm.com, seiden@linux.ibm.com, nrb@linux.ibm.com
Subject: Re: [PATCH v9 3/3] s390x: KVM: resetting the Topology-Change-Report
Message-ID: <20220512115250.2e20bfdf@p-imbrenda>
In-Reply-To: <76fd0c11-5b9b-0032-183b-54db650f13b1@redhat.com>
References: <20220506092403.47406-1-pmorel@linux.ibm.com>
        <20220506092403.47406-4-pmorel@linux.ibm.com>
        <76fd0c11-5b9b-0032-183b-54db650f13b1@redhat.com>
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On Thu, 12 May 2022 11:31:18 +0200
David Hildenbrand <david@redhat.com> wrote:

> On 06.05.22 11:24, 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, let's give userland the possibility to
> > query the MTCR state.
> > 
> > Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> > ---
> >  arch/s390/include/uapi/asm/kvm.h |  5 ++
> >  arch/s390/kvm/kvm-s390.c         | 79 ++++++++++++++++++++++++++++++++
> >  2 files changed, 84 insertions(+)
> > 
> > diff --git a/arch/s390/include/uapi/asm/kvm.h b/arch/s390/include/uapi/asm/kvm.h
> > index 7a6b14874d65..abdcf4069343 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
> > @@ -171,6 +172,10 @@ struct kvm_s390_vm_cpu_subfunc {
> >  #define KVM_S390_VM_MIGRATION_START	1
> >  #define KVM_S390_VM_MIGRATION_STATUS	2
> >  
> > +/* kvm attributes for cpu topology */
> > +#define KVM_S390_VM_CPU_TOPO_MTR_CLEAR	0
> > +#define KVM_S390_VM_CPU_TOPO_MTR_SET	1
> > +
> >  /* for KVM_GET_REGS and KVM_SET_REGS */
> >  struct kvm_regs {
> >  	/* general purpose regs for s390 */
> > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> > index c8bdce31464f..80a1244f0ead 100644
> > --- a/arch/s390/kvm/kvm-s390.c
> > +++ b/arch/s390/kvm/kvm-s390.c
> > @@ -1731,6 +1731,76 @@ static void kvm_s390_sca_set_mtcr(struct kvm *kvm)
> >  	ipte_unlock(kvm);
> >  }
> >  
> > +/**
> > + * kvm_s390_sca_clear_mtcr
> > + * @kvm: guest KVM description
> > + *
> > + * Is only relevant if the topology facility is present,
> > + * the caller should check KVM facility 11
> > + *
> > + * Updates the Multiprocessor Topology-Change-Report to signal
> > + * the guest with a topology change.
> > + */
> > +static void kvm_s390_sca_clear_mtcr(struct kvm *kvm)
> > +{
> > +	struct bsca_block *sca = kvm->arch.sca; /* SCA version doesn't matter */
> > +
> > +	ipte_lock(kvm);
> > +	sca->utility  &= ~SCA_UTILITY_MTCR;  
> 
> 
> One space too much.
> 
> sca->utility &= ~SCA_UTILITY_MTCR;
> 
> > +	ipte_unlock(kvm);
> > +}
> > +
> > +static int kvm_s390_set_topology(struct kvm *kvm, struct kvm_device_attr *attr)
> > +{
> > +	if (!test_kvm_facility(kvm, 11))
> > +		return -ENXIO;
> > +
> > +	switch (attr->attr) {
> > +	case KVM_S390_VM_CPU_TOPO_MTR_SET:
> > +		kvm_s390_sca_set_mtcr(kvm);
> > +		break;
> > +	case KVM_S390_VM_CPU_TOPO_MTR_CLEAR:
> > +		kvm_s390_sca_clear_mtcr(kvm);
> > +		break;
> > +	}
> > +	return 0;
> > +}
> > +
> > +/**
> > + * kvm_s390_sca_get_mtcr
> > + * @kvm: guest KVM description
> > + *
> > + * Is only relevant if the topology facility is present,
> > + * the caller should check KVM facility 11
> > + *
> > + * reports to QEMU the Multiprocessor Topology-Change-Report.
> > + */
> > +static int kvm_s390_sca_get_mtcr(struct kvm *kvm)
> > +{
> > +	struct bsca_block *sca = kvm->arch.sca; /* SCA version doesn't matter */
> > +	int val;
> > +
> > +	ipte_lock(kvm);
> > +	val = !!(sca->utility & SCA_UTILITY_MTCR);
> > +	ipte_unlock(kvm);
> > +
> > +	return val;
> > +}
> > +
> > +static int kvm_s390_get_topology(struct kvm *kvm, struct kvm_device_attr *attr)
> > +{
> > +	int mtcr;  
> 
> I think we prefer something like u16 when copying to user space.

but then userspace also has to expect a u16, right?

> 
> > +
> > +	if (!test_kvm_facility(kvm, 11))
> > +		return -ENXIO;
> > +
> > +	mtcr = kvm_s390_sca_get_mtcr(kvm);
> > +	if (copy_to_user((void __user *)attr->addr, &mtcr, sizeof(mtcr)))
> > +		return -EFAULT;
> > +
> > +	return 0;
> > +}  
> 
> You should probably add documentation, and document that only the last
> bit (0x1) has a meaning.
> 
> Apart from that LGTM.
>