Re: [PATCH v17 6/6] KVM: arm64: Document MTE capability and ioctl

[Fuad Tabba]

Hi,


On Mon, Jun 21, 2021 at 12:18 PM Steven Price <steven.price@xxxxxxx> 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.
>
> Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> Signed-off-by: Steven Price <steven.price@xxxxxxx>
> ---
>  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 7fcb2fd38f42..97661a97943f 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.

I was wondering if there might be an issue with AArch32 at EL0 and
MTE, because I think that even if AArch64 at EL1 is disallowed, the
guest can still run AArch32 at EL0.

Thanks,
/fuad

> +
> +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 memslots must be mapped as
> +not-shareable (no MAP_SHARED), attempts to create a memslot with a
> +MAP_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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