vCPU feature selection on arm64

KVM/arm64 provides two mechanisms that allow userspace to configure the CPU features presented to the guest.


The KVM_ARM_VCPU_INIT ioctl accepts a bitmap of feature flags (struct kvm_vcpu_init::features). Features enabled by this interface are opt-in and may change/extend UAPI. See 4.82 KVM_ARM_VCPU_INIT for complete documentation of the features controlled by the ioctl.

Otherwise, all CPU features supported by KVM are described by the architected ID registers.

The ID Registers

The Arm architecture specifies a range of ID Registers that describe the set of architectural features supported by the CPU implementation. KVM initializes the guest’s ID registers to the maximum set of CPU features supported by the system. The ID register values may be VM-scoped in KVM, meaning that the values could be shared for all vCPUs in a VM.

KVM allows userspace to opt-out of certain CPU features described by the ID registers by writing values to them via the KVM_SET_ONE_REG ioctl. The ID registers are mutable until the VM has started, i.e. userspace has called KVM_RUN on at least one vCPU in the VM. Userspace can discover what fields are mutable in the ID registers using the KVM_ARM_GET_REG_WRITABLE_MASKS. See the ioctl documentation for more details.

Userspace is allowed to limit or mask CPU features according to the rules outlined by the architecture in DDI0487J.a D19.1.3 ‘Principles of the ID scheme for fields in ID register’. KVM does not allow ID register values that exceed the capabilities of the system.


It is strongly recommended that userspace modify the ID register values before accessing the rest of the vCPU’s CPU register state. KVM may use the ID register values to control feature emulation. Interleaving ID register modification with other system register accesses may lead to unpredictable behavior.