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    Subject[PATCH v16 7/7] KVM: arm64: Document MTE capability and ioctl
    Date
    A new capability (KVM_CAP_ARM_MTE) identifies that the kernel supports
    granting a guest access to the tags, and provides a mechanism for the
    VMM to enable it.

    A new ioctl (KVM_ARM_MTE_COPY_TAGS) provides a simple way for a VMM to
    access the tags of a guest without having to maintain a PROT_MTE mapping
    in userspace. The above capability gates access to the ioctl.

    Signed-off-by: Steven Price <steven.price@arm.com>
    ---
    Documentation/virt/kvm/api.rst | 61 ++++++++++++++++++++++++++++++++++
    1 file changed, 61 insertions(+)

    diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
    index 22d077562149..3c27e712b1fb 100644
    --- a/Documentation/virt/kvm/api.rst
    +++ b/Documentation/virt/kvm/api.rst
    @@ -5034,6 +5034,43 @@ see KVM_XEN_VCPU_SET_ATTR above.
    The KVM_XEN_VCPU_ATTR_TYPE_RUNSTATE_ADJUST type may not be used
    with the KVM_XEN_VCPU_GET_ATTR ioctl.

    +4.130 KVM_ARM_MTE_COPY_TAGS
    +---------------------------
    +
    +:Capability: KVM_CAP_ARM_MTE
    +:Architectures: arm64
    +:Type: vm ioctl
    +:Parameters: struct kvm_arm_copy_mte_tags
    +:Returns: number of bytes copied, < 0 on error (-EINVAL for incorrect
    + arguments, -EFAULT if memory cannot be accessed).
    +
    +::
    +
    + struct kvm_arm_copy_mte_tags {
    + __u64 guest_ipa;
    + __u64 length;
    + void __user *addr;
    + __u64 flags;
    + __u64 reserved[2];
    + };
    +
    +Copies Memory Tagging Extension (MTE) tags to/from guest tag memory. The
    +``guest_ipa`` and ``length`` fields must be ``PAGE_SIZE`` aligned. The ``addr``
    +field must point to a buffer which the tags will be copied to or from.
    +
    +``flags`` specifies the direction of copy, either ``KVM_ARM_TAGS_TO_GUEST`` or
    +``KVM_ARM_TAGS_FROM_GUEST``.
    +
    +The size of the buffer to store the tags is ``(length / 16)`` bytes
    +(granules in MTE are 16 bytes long). Each byte contains a single tag
    +value. This matches the format of ``PTRACE_PEEKMTETAGS`` and
    +``PTRACE_POKEMTETAGS``.
    +
    +If an error occurs before any data is copied then a negative error code is
    +returned. If some tags have been copied before an error occurs then the number
    +of bytes successfully copied is returned. If the call completes successfully
    +then ``length`` is returned.
    +
    5. The kvm_run structure
    ========================

    @@ -6362,6 +6399,30 @@ default.

    See Documentation/x86/sgx/2.Kernel-internals.rst for more details.

    +7.26 KVM_CAP_ARM_MTE
    +--------------------
    +
    +:Architectures: arm64
    +:Parameters: none
    +
    +This capability indicates that KVM (and the hardware) supports exposing the
    +Memory Tagging Extensions (MTE) to the guest. It must also be enabled by the
    +VMM before creating any VCPUs to allow the guest access. Note that MTE is only
    +available to a guest running in AArch64 mode and enabling this capability will
    +cause attempts to create AArch32 VCPUs to fail.
    +
    +When enabled the guest is able to access tags associated with any memory given
    +to the guest. KVM will ensure that the tags are maintained during swap or
    +hibernation of the host; however the VMM needs to manually save/restore the
    +tags as appropriate if the VM is migrated.
    +
    +When this capability is enabled all memory in (non-device) memslots must not
    +used VM_SHARED, attempts to create a memslot with a VM_SHARED mmap will result
    +in an -EINVAL return.
    +
    +When enabled the VMM may make use of the ``KVM_ARM_MTE_COPY_TAGS`` ioctl to
    +perform a bulk copy of tags to/from the guest.
    +
    8. Other capabilities.
    ======================

    --
    2.20.1
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