1.. SPDX-License-Identifier: GPL-2.0 2 3=============================== 4vCPU feature selection on arm64 5=============================== 6 7KVM/arm64 provides two mechanisms that allow userspace to configure 8the CPU features presented to the guest. 9 10KVM_ARM_VCPU_INIT 11================= 12 13The ``KVM_ARM_VCPU_INIT`` ioctl accepts a bitmap of feature flags 14(``struct kvm_vcpu_init::features``). Features enabled by this interface are 15*opt-in* and may change/extend UAPI. See :ref:`KVM_ARM_VCPU_INIT` for complete 16documentation of the features controlled by the ioctl. 17 18Otherwise, all CPU features supported by KVM are described by the architected 19ID registers. 20 21The ID Registers 22================ 23 24The Arm architecture specifies a range of *ID Registers* that describe the set 25of architectural features supported by the CPU implementation. KVM initializes 26the guest's ID registers to the maximum set of CPU features supported by the 27system. The ID register values may be VM-scoped in KVM, meaning that the 28values could be shared for all vCPUs in a VM. 29 30KVM allows userspace to *opt-out* of certain CPU features described by the ID 31registers by writing values to them via the ``KVM_SET_ONE_REG`` ioctl. The ID 32registers are mutable until the VM has started, i.e. userspace has called 33``KVM_RUN`` on at least one vCPU in the VM. Userspace can discover what fields 34are 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 more 36details. 37 38Userspace is allowed to *limit* or *mask* CPU features according to the rules 39outlined by the architecture in DDI0487J.a D19.1.3 'Principles of the ID 40scheme for fields in ID register'. KVM does not allow ID register values that 41exceed the capabilities of the system. 42 43.. warning:: 44 It is **strongly recommended** that userspace modify the ID register values 45 before accessing the rest of the vCPU's CPU register state. KVM may use the 46 ID register values to control feature emulation. Interleaving ID register 47 modification with other system register accesses may lead to unpredictable 48 behavior. 49