[PATCH v9 6/6] KVM: arm64: Document MTE capability and ioctl

[Steven Price]

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 at arm.com>
---
 Documentation/virt/kvm/api.rst | 37 ++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index aed52b0fc16e..1406ea138127 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -4939,6 +4939,23 @@ KVM_XEN_VCPU_ATTR_TYPE_VCPU_TIME_INFO
 Allows Xen vCPU attributes to be read. For the structure and types,
 see KVM_XEN_VCPU_SET_ATTR above.
 
+4.131 KVM_ARM_MTE_COPY_TAGS
+---------------------------
+
+:Capability: KVM_CAP_ARM_MTE
+:Architectures: arm64
+:Type: vm ioctl
+:Parameters: struct kvm_arm_copy_mte_tags
+:Returns: 0 on success, < 0 on error
+
+Copies Memory Tagging Extension (MTE) tags to/from guest tag memory. The
+starting address and length of guest memory must be ``PAGE_SIZE`` aligned.
+
+The size of the buffer to store the tags is ``(length / MTE_GRANULE_SIZE)``
+bytes (i.e. 1/16th of the corresponding size). Each byte contains a single tag
+value. This matches the format of ``PTRACE_PEEKMTETAGS`` and
+``PTRACE_POKEMTETAGS``.
+
 5. The kvm_run structure
 ========================
 
@@ -6227,6 +6244,25 @@ KVM_RUN_BUS_LOCK flag is used to distinguish between them.
 This capability can be used to check / enable 2nd DAWR feature provided
 by POWER10 processor.
 
+7.23 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 the guest will be granted access.
+
+When enabled the guest is able to access tags associated with any memory given
+to the guest. KVM will ensure that the pages are flagged ``PG_mte_tagged`` so
+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 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.
 ======================
 
@@ -6716,3 +6752,4 @@ KVM_XEN_HVM_SET_ATTR, KVM_XEN_HVM_GET_ATTR, KVM_XEN_VCPU_SET_ATTR and
 KVM_XEN_VCPU_GET_ATTR ioctls, as well as the delivery of exception vectors
 for event channel upcalls when the evtchn_upcall_pending field of a vcpu's
 vcpu_info is set.
+
-- 
2.20.1