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1.. SPDX-License-Identifier: GPL-2.02 3===============================4vCPU feature selection on arm645===============================6 7KVM/arm64 provides two mechanisms that allow userspace to configure8the CPU features presented to the guest.9 10KVM_ARM_VCPU_INIT11=================12 13The ``KVM_ARM_VCPU_INIT`` ioctl accepts a bitmap of feature flags14(``struct kvm_vcpu_init::features``). Features enabled by this interface are15*opt-in* and may change/extend UAPI. See :ref:`KVM_ARM_VCPU_INIT` for complete16documentation of the features controlled by the ioctl.17 18Otherwise, all CPU features supported by KVM are described by the architected19ID registers.20 21The ID Registers22================23 24The Arm architecture specifies a range of *ID Registers* that describe the set25of architectural features supported by the CPU implementation. KVM initializes26the guest's ID registers to the maximum set of CPU features supported by the27system. The ID register values may be VM-scoped in KVM, meaning that the28values could be shared for all vCPUs in a VM.29 30KVM allows userspace to *opt-out* of certain CPU features described by the ID31registers by writing values to them via the ``KVM_SET_ONE_REG`` ioctl. The ID32registers are mutable until the VM has started, i.e. userspace has called33``KVM_RUN`` on at least one vCPU in the VM. Userspace can discover what fields34are mutable in the ID registers using the ``KVM_ARM_GET_REG_WRITABLE_MASKS``.35See the :ref:`ioctl documentation <KVM_ARM_GET_REG_WRITABLE_MASKS>` for more36details.37 38Userspace is allowed to *limit* or *mask* CPU features according to the rules39outlined by the architecture in DDI0487J.a D19.1.3 'Principles of the ID40scheme for fields in ID register'. KVM does not allow ID register values that41exceed the capabilities of the system.42 43.. warning::44 It is **strongly recommended** that userspace modify the ID register values45 before accessing the rest of the vCPU's CPU register state. KVM may use the46 ID register values to control feature emulation. Interleaving ID register47 modification with other system register accesses may lead to unpredictable48 behavior.49