On 17/05/2021 19:09, Marc Zyngier wrote:

On Mon, 17 May 2021 13:32:39 +0100,
Steven Price [off-list ref] wrote:

quoted

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 | 53 ++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 22d077562149..a31661b870ba 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst

@@ -5034,6 +5034,40 @@ 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: 0 on success, < 0 on error
+
+::
+
+  struct kvm_arm_copy_mte_tags {
+	__u64 guest_ipa;
+	__u64 length;
+	union {
+		void __user *addr;
+		__u64 padding;
+	};
+	__u64 flags;
+	__u64 reserved[2];
+  };

This doesn't exactly match the structure in the previous patch :-(.

:( I knew there was a reason I didn't include it in the documentation
for the first 9 versions... I'll fix this up, thanks for spotting it.

quoted

+
+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 / MTE_GRANULE_SIZE)``

Should we add a UAPI definition for MTE_GRANULE_SIZE?

I wasn't sure whether to export this or not. The ioctl is based around
the existing ptrace interface (PTRACE_{PEEK,POKE}MTETAGS) which doesn't
expose a UAPI definition. Admittedly the documentation there also just
says "16-byte granule" rather than MTE_GRANULE_SIZE.

So I'll just remove the reference to MTE_GRANULE_SIZE in the
documentation unless you feel that we should have a UAPI definition.

quoted

+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
 ========================
 

@@ -6362,6 +6396,25 @@ 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 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.
+

Missing limitation to AArch64 guests.

As mentioned previously it's not technically limited to AArch64, but
I'll expand this to make it clear that MTE isn't usable from a AArch32 VCPU.

Thanks,

Steve

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