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    SubjectRe: [PATCH v17 6/6] KVM: arm64: Document MTE capability and ioctl
    Hi,


    On Mon, Jun 21, 2021 at 12:18 PM Steven Price <steven.price@arm.com> 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@arm.com>
    > Signed-off-by: Steven Price <steven.price@arm.com>
    > ---
    > 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
    >
    > _______________________________________________
    > kvmarm mailing list
    > kvmarm@lists.cs.columbia.edu
    > https://lists.cs.columbia.edu/mailman/listinfo/kvmarm

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