Documentation/virt/kvm/arm/fw-pseudo-registers.rst

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Documentation/virt/kvm/arm/fw-pseudo-registers.rst
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Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

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.. SPDX-License-Identifier: GPL-2.0

=======================================
ARM firmware pseudo-registers interface
=======================================

KVM handles the hypercall services as requested by the guests. New hypercall
services are regularly made available by the ARM specification or by KVM (as
vendor services) if they make sense from a virtualization point of view.

This means that a guest booted on two different versions of KVM can observe
two different "firmware" revisions. This could cause issues if a given guest
is tied to a particular version of a hypercall service, or if a migration
causes a different version to be exposed out of the blue to an unsuspecting
guest.

In order to remedy this situation, KVM exposes a set of "firmware
pseudo-registers" that can be manipulated using the GET/SET_ONE_REG
interface. These registers can be saved/restored by userspace, and set
to a convenient value as required.

The following registers are defined:

* KVM_REG_ARM_PSCI_VERSION:

  KVM implements the PSCI (Power State Coordination Interface)
  specification in order to provide services such as CPU on/off, reset
  and power-off to the guest.

  - Only valid if the vcpu has the KVM_ARM_VCPU_PSCI_0_2 feature set
    (and thus has already been initialized)
  - Returns the current PSCI version on GET_ONE_REG (defaulting to the
    highest PSCI version implemented by KVM and compatible with v0.2)
  - Allows any PSCI version implemented by KVM and compatible with
    v0.2 to be set with SET_ONE_REG
  - Affects the whole VM (even if the register view is per-vcpu)

* KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1:
    Holds the state of the firmware support to mitigate CVE-2017-5715, as
    offered by KVM to the guest via a HVC call. The workaround is described
    under SMCCC_ARCH_WORKAROUND_1 in [1].

  Accepted values are:

    KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1_NOT_AVAIL:
      KVM does not offer
      firmware support for the workaround. The mitigation status for the
      guest is unknown.
    KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1_AVAIL:
      The workaround HVC call is
      available to the guest and required for the mitigation.
    KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1_NOT_REQUIRED:
      The workaround HVC call
      is available to the guest, but it is not needed on this VCPU.

* KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2:
    Holds the state of the firmware support to mitigate CVE-2018-3639, as
    offered by KVM to the guest via a HVC call. The workaround is described
    under SMCCC_ARCH_WORKAROUND_2 in [1]_.

  Accepted values are:

    KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_NOT_AVAIL:
      A workaround is not
      available. KVM does not offer firmware support for the workaround.
    KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_UNKNOWN:
      The workaround state is
      unknown. KVM does not offer firmware support for the workaround.
    KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_AVAIL:
      The workaround is available,

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