linux-kernel - Re: [PATCH v8 2/2] s390x: KVM: resetting the Topology-Change-Report

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Date:   Thu, 28 Apr 2022 15:50:54 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Pierre Morel <pmorel@...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,
        thuth@...hat.com, imbrenda@...ux.ibm.com, hca@...ux.ibm.com,
        gor@...ux.ibm.com, wintera@...ux.ibm.com, seiden@...ux.ibm.com,
        nrb@...ux.ibm.com
Subject: Re: [PATCH v8 2/2] s390x: KVM: resetting the Topology-Change-Report

On 20.04.22 13:34, 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@...ux.ibm.com>
> ---
>  arch/s390/include/uapi/asm/kvm.h |   9 +++
>  arch/s390/kvm/kvm-s390.c         | 103 +++++++++++++++++++++++++++++++
>  2 files changed, 112 insertions(+)
> 
> diff --git a/arch/s390/include/uapi/asm/kvm.h b/arch/s390/include/uapi/asm/kvm.h
> index 7a6b14874d65..bb3df6d49f27 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 +172,14 @@ 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
> +
> +struct kvm_s390_cpu_topology {
> +	__u16 mtcr;
> +};

Just wondering:

1) Do we really need a struct for that
2) Do we want to leave some room for later expansion?

> +
>  /* 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 925ccc59f283..755f325c9e70 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -1756,6 +1756,100 @@ static int kvm_s390_sca_set_mtcr(struct kvm *kvm)
>  	return 0;
>  }
>  
> +/**
> + * 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 int kvm_s390_sca_clear_mtcr(struct kvm *kvm)
> +{
> +	struct bsca_block *sca = kvm->arch.sca;
> +	struct kvm_vcpu *vcpu;
> +	int val;
> +
> +	vcpu = kvm_s390_get_first_vcpu(kvm);
> +	if (!vcpu)
> +		return -ENODEV;

It would be cleaner to have ipte_lock/ipte_unlock variants that are
independent of a vcpu.

Instead of checking for "vcpu->arch.sie_block->eca & ECA_SII" we might
just check for sclp.has_siif. Everything else that performs the
lock/unlock should be contained in "struct kvm" directly, unless I am
missing something.

[...]

> +
> +static int kvm_s390_get_topology(struct kvm *kvm, struct kvm_device_attr *attr)
> +{
> +	struct kvm_s390_cpu_topology *topology;
> +	int ret = 0;
> +
> +	if (!test_kvm_facility(kvm, 11))
> +		return -ENXIO;
> +
> +	topology = kzalloc(sizeof(*topology), GFP_KERNEL);
> +	if (!topology)
> +		return -ENOMEM;

I'm confused. We're allocating a __u16 to then free it again below? Why
not simply use a value on the stack like in kvm_s390_vm_get_migration()?



u16 mtcr;
...
mtcr = kvm_s390_sca_get_mtcr(kvm);

if (copy_to_user((void __user *)attr->addr, &mtcr, sizeof(mtcr)))
	return -EFAULT;
return 0;



> +
> +	topology->mtcr =  kvm_s390_sca_get_mtcr(kvm);

s/  / /

> +	if (copy_to_user((void __user *)attr->addr, topology,
> +			 sizeof(struct kvm_s390_cpu_topology)))
> +		ret = -EFAULT;
> +
> +	kfree(topology);
> +	return ret;
> +}
> +


-- 
Thanks,

David / dhildenb