Re: [PATCH v4 1/2] arm/kvm: add support for MTE

[Peter Maydell]

On Wed, 11 Jan 2023 at 16:13, Cornelia Huck <cohuck@redhat.com> wrote:
>
> Introduce a new cpu feature flag to control MTE support. To preserve
> backwards compatibility for tcg, MTE will continue to be enabled as
> long as tag memory has been provided.
>
> If MTE has been enabled, we need to disable migration, as we do not
> yet have a way to migrate the tags as well. Therefore, MTE will stay
> off with KVM unless requested explicitly.
>
> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> ---
>  docs/system/arm/cpu-features.rst |  21 +++++
>  hw/arm/virt.c                    |   2 +-
>  target/arm/cpu.c                 |  18 ++---
>  target/arm/cpu.h                 |   1 +
>  target/arm/cpu64.c               | 133 +++++++++++++++++++++++++++++++
>  target/arm/internals.h           |   1 +
>  target/arm/kvm64.c               |   5 ++
>  target/arm/kvm_arm.h             |  12 +++
>  target/arm/monitor.c             |   1 +
>  9 files changed, 181 insertions(+), 13 deletions(-)
>
> diff --git a/docs/system/arm/cpu-features.rst 
> b/docs/system/arm/cpu-features.rst
> index 00c444042ff5..e278650c837e 100644
> --- a/docs/system/arm/cpu-features.rst
> +++ b/docs/system/arm/cpu-features.rst
> @@ -443,3 +443,24 @@ As with ``sve-default-vector-length``, if the default 
> length is larger
>  than the maximum vector length enabled, the actual vector length will
>  be reduced.  If this property is set to ``-1`` then the default vector
>  length is set to the maximum possible length.
> +
> +MTE CPU Property
> +================
> +
> +The ``mte`` property controls the Memory Tagging Extension. For TCG, it 
> requires
> +presence of tag memory (which can be turned on for the ``virt`` machine via
> +``mte=on``). For KVM, it requires the ``KVM_CAP_ARM_MTE`` capability; until
> +proper migration support is implemented, enabling MTE will install a 
> migration
> +blocker.
> +
> +If not specified explicitly via ``on`` or ``off``, MTE will be available
> +according to the following rules:
> +
> +* When TCG is used, MTE will be available iff tag memory is available; i.e. 
> it
> +  preserves the behaviour prior to introduction of the feature.
> +
> +* When KVM is used, MTE will default to off, so that migration will not
> +  unintentionally be blocked.
> +
> +* Other accelerators currently don't support MTE.

Minor nits for the documentation:
we should expand out "if and only if" -- not everybody recognizes
"iff", especially if they're not native English speakers or not
mathematicians.

Should we write specifically that in a future QEMU version KVM
might change to defaulting to "on if available" when migration
support is implemented?

thanks
-- PMM