[PATCH v14 8/8] 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@xxxxxxx>
---
 Documentation/virt/kvm/api.rst | 57 ++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 22d077562149..fc6f0cbc30b3 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -5034,6 +5034,42 @@ 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
+
+::
+
+  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``
+fieldmust 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 +6398,27 @@ 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 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.
 ======================
 
-- 
2.20.1