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diff --git a/Documentation/virt/kvm/arm/vcpu-features.rst b/Documentation/virt/kvm/arm/vcpu-features.rst new file mode 100644 index 0000000000..f7cc6d8d8b --- /dev/null +++ b/Documentation/virt/kvm/arm/vcpu-features.rst @@ -0,0 +1,48 @@ +.. 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. |