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

[Fuad Tabba <tabba@google.com>]

Hi Marc,

On Tue, Jun 22, 2021 at 11:35 AM Marc Zyngier <maz@kernel.org> wrote:
>
> On Tue, 22 Jun 2021 10:42:42 +0100,
> Fuad Tabba <tabba@google.com> wrote:
> >
> > 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
>
> Did you mean AArch32 here?

Yes.

> > guest can still run AArch32 at EL0.
>
> I don't get your question:
>
> - If the guest is AArch32 at EL1, there is not MTE whatsoever (where
>   would you place the tag?)
>
> - If the guest is AArch64, it can have MTE enabled or not,
>   irrespective of the EL. If this guest decides to run an AArch32 EL0,
>   the architecture rules still apply, and it cannot expose MTE to its
>   own 32bit userspace. Nothing that KVM needs to do about this.
>
> What KVM enforces is that at the point where the guest is in charge,
> we have a consistent architectural behaviour.

This answers my question. I was wondering whether we should be
concerned with the case where the guest decides to run an AArch32 EL0.

Thanks,
/fuad

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

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