Re: [libvirt] [RFC v2] arm64: KVM: KVM API extensions for SVE

Vector length control: Some means is needed to determine the set of vector lengths visible to guest software running on a vcpu. When a vcpu is created, the set would be defaulted to the maximal set that can be supported while permitting each vcpu to run on any host CPU. SVE has some virtualisation quirks which mean that this set may exclude some vector lengths that are available for host userspace applications. The common case should be that the sets are the same however. * New ioctl KVM_ARM_VCPU_{SET,GET}_SVE_VLS to set or retrieve the set of vector lengths available to the guest. Adding random vcpu ioctls To configure a non-default set of vector lengths, KVM_ARM_VCPU_SET_SVE_VLS can be called: this would only be permitted before the vcpu is first run. This is primarily intended for supporting migration, by providing a robust check that the destination node will run the vcpu correctly. In a cluster with non-uniform SVE implementation across nodes, this also allows a specific set of VLs to be requested that the caller knows is usable across the whole cluster. For migration purposes, userspace would need to do KVM_ARM_VCPU_GET_SVE_VLS at the origin node and store the returned set as VM metadata: on the destination node, KVM_ARM_VCPU_SET_SVE_VLS should be used to request that exact set of VLs: if the destination node can't support that set of VLs, the call will fail.