1 .. SPDX-License-Identifier: GPL-2.0 2 3 =============================== 4 vCPU feature selection on arm64 5 =============================== 6 7 KVM/arm64 provides two mechanisms that allow userspace to configure 8 the CPU features presented to the guest. 9 10 KVM_ARM_VCPU_INIT 11 ================= 12 13 The ``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 16 documentation of the features controlled by the ioctl. 17 18 Otherwise, all CPU features supported by KVM are described by the architected 19 ID registers. 20 21 The ID Registers 22 ================ 23 24 The Arm architecture specifies a range of *ID Registers* that describe the set 25 of architectural features supported by the CPU implementation. KVM initializes 26 the guest's ID registers to the maximum set of CPU features supported by the 27 system. The ID register values may be VM-scoped in KVM, meaning that the 28 values could be shared for all vCPUs in a VM. 29 30 KVM allows userspace to *opt-out* of certain CPU features described by the ID 31 registers by writing values to them via the ``KVM_SET_ONE_REG`` ioctl. The ID 32 registers 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 34 are mutable in the ID registers using the ``KVM_ARM_GET_REG_WRITABLE_MASKS``. 35 See the :ref:`ioctl documentation <KVM_ARM_GET_REG_WRITABLE_MASKS>` for more 36 details. 37 38 Userspace is allowed to *limit* or *mask* CPU features according to the rules 39 outlined by the architecture in DDI0487J.a D19.1.3 'Principles of the ID 40 scheme for fields in ID register'. KVM does not allow ID register values that 41 exceed 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.
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