dafa493dd0
KVM/arm64 has a couple schemes for handling vCPU feature selection now, which is a lot to put on userspace. Add some documentation about how these interact and provide some recommendations for how to use the writable ID register scheme. Reviewed-by: Marc Zyngier <maz@kernel.org> Link: https://lore.kernel.org/r/20231003230408.3405722-11-oliver.upton@linux.dev Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
49 lines
2.1 KiB
ReStructuredText
49 lines
2.1 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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===============================
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vCPU feature selection on arm64
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===============================
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KVM/arm64 provides two mechanisms that allow userspace to configure
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the CPU features presented to the guest.
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KVM_ARM_VCPU_INIT
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=================
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The ``KVM_ARM_VCPU_INIT`` ioctl accepts a bitmap of feature flags
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(``struct kvm_vcpu_init::features``). Features enabled by this interface are
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*opt-in* and may change/extend UAPI. See :ref:`KVM_ARM_VCPU_INIT` for complete
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documentation of the features controlled by the ioctl.
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Otherwise, all CPU features supported by KVM are described by the architected
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ID registers.
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The ID Registers
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================
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The Arm architecture specifies a range of *ID Registers* that describe the set
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of architectural features supported by the CPU implementation. KVM initializes
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the guest's ID registers to the maximum set of CPU features supported by the
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system. The ID register values may be VM-scoped in KVM, meaning that the
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values could be shared for all vCPUs in a VM.
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KVM allows userspace to *opt-out* of certain CPU features described by the ID
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registers by writing values to them via the ``KVM_SET_ONE_REG`` ioctl. The ID
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registers are mutable until the VM has started, i.e. userspace has called
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``KVM_RUN`` on at least one vCPU in the VM. Userspace can discover what fields
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are mutable in the ID registers using the ``KVM_ARM_GET_REG_WRITABLE_MASKS``.
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See the :ref:`ioctl documentation <KVM_ARM_GET_REG_WRITABLE_MASKS>` for more
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details.
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Userspace is allowed to *limit* or *mask* CPU features according to the rules
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outlined by the architecture in DDI0487J.a D19.1.3 'Principles of the ID
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scheme for fields in ID register'. KVM does not allow ID register values that
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exceed the capabilities of the system.
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.. warning::
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It is **strongly recommended** that userspace modify the ID register values
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before accessing the rest of the vCPU's CPU register state. KVM may use the
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ID register values to control feature emulation. Interleaving ID register
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modification with other system register accesses may lead to unpredictable
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behavior.
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