.. SPDX-License-Identifier: GPL-2.0

===============================
vCPU feature selection on arm64
===============================

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

KVM_ARM_VCPU_INIT
=================

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 :ref:`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 :ref:`ioctl documentation <KVM_ARM_GET_REG_WRITABLE_MASKS>` 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.

.. warning::
   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.