Subject: Re: [PATCH v12 8/8] KVM: arm64: Document MTE capability and ioctl

From: Marc Zyngier <[email-masked]>
Date: Mon, 17 May 2021 19:09:37 +0100

On Mon, 17 May 2021 13:32:39 +0100,
Steven Price <[email-masked]> wrote:
> 
> 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 <[email-masked]>
> ---
>  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 :-(.

> +
> +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?

> +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.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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