Re: [PATCH v6 1/2] arm/kvm: add support for MTE

[Andrea Bolognani]

On Tue, Feb 28, 2023 at 04:02:15PM +0100, Cornelia Huck 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               | 110 +++++++++++++++++++++++++++++++
>  target/arm/internals.h           |   1 +
>  target/arm/kvm.c                 |  29 ++++++++
>  target/arm/kvm64.c               |   5 ++
>  target/arm/kvm_arm.h             |  19 ++++++
>  target/arm/monitor.c             |   1 +
>  10 files changed, 194 insertions(+), 13 deletions(-)

I've given a quick look with libvirt integration in mind, and
everything seem fine.

Specifically, MTE is advertised in the output of qom-list-properties
both for max-arm-cpu and the latest virt-X.Y-machine, which means
that libvirt can easily and reliably figure out whether MTE support
is available.

> +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.

Is it okay to use -machine virt,mte=on unconditionally for both KVM
and TCG guests when MTE support is requested, or will that not work
for the former?

> +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 if and only if tag memory is 
> available;
> +  i.e. it preserves the behaviour prior to the introduction of the feature.
> +
> +* When KVM is used, MTE will default to off, so that migration will not
> +  unintentionally be blocked. This might change in a future QEMU version.

If and when this changes, we should ensure that the new default
behavior doesn't affect existing machine types, otherwise we will
break guest ABI for existing VMs.

-- 
Andrea Bolognani / Red Hat / Virtualization