Re: [RFC/PATCH v2 21/22] KVM: s390: Add KVM HPAGE capability

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 13 Dec 2017 13:53:32 +0100
Janosch Frank <frankja@linux.vnet.ibm.com> wrote:

> KVM huge page backing support can not be easily tested under
> s390. Currently testing is only possible after most of the guest has
> already been set up.
> 
> To indicate, that KVM has huge page backing support, we add the
> KVM_CAP_S390_HPAGE capability. This does not mean, that transparent
> huge pages are supported.

Do you expect to use a different cap for non-1MB huge pages? If yes,
this should probably be mentioned here.

> 
> Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
> ---
>  Documentation/virtual/kvm/api.txt | 10 ++++++++++
>  arch/s390/kvm/kvm-s390.c          |  1 +
>  include/uapi/linux/kvm.h          |  1 +
>  3 files changed, 12 insertions(+)
> 
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 57d3ee9..a56b0af 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -4369,3 +4369,13 @@ Parameters: none
>  This capability indicates if the flic device will be able to get/set the
>  AIS states for migration via the KVM_DEV_FLIC_AISM_ALL attribute and allows
>  to discover this without having to create a flic device.
> +
> +8.14 KVM_CAP_S390_HPAGE
> +
> +Architectures: s390
> +This capability, if KVM_CHECK_EXTENSION indicates that it is
> +available, means that KVM supports VMs that are memory backed through
> +hugetlbfs with 1 megabyte pages.
> +
> +While it is generally possible to create and start a VM without this
> +support, the VM will not be functional.

This sentence applies only to the hugepage case, doesn't it?

> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 73fb3bc..8951ad4 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -393,6 +393,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  	case KVM_CAP_S390_CMMA_MIGRATION:
>  	case KVM_CAP_S390_AIS:
>  	case KVM_CAP_S390_AIS_MIGRATION:
> +	case KVM_CAP_S390_HPAGE:
>  		r = 1;
>  		break;
>  	case KVM_CAP_S390_MEM_OP:
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 496e59a..aa3d707 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -932,6 +932,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_HYPERV_SYNIC2 148
>  #define KVM_CAP_HYPERV_VP_INDEX 149
>  #define KVM_CAP_S390_AIS_MIGRATION 150
> +#define KVM_CAP_S390_HPAGE 151
>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>