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

[Cornelia Huck <cohuck@redhat.com>]

On Tue, Feb 28 2023, Andrea Bolognani <abologna@redhat.com> wrote:

> 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.

Great, thanks for having a look!

>
>> +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?

QEMU will error out if you try this with KVM (basically, same behaviour
as before.) Is that a problem for libvirt, or merely a bit inconvinient?

>
>> +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.

Nod, such a change would need proper compat handling. It's not quite
clear yet if we'll ever flip it, though.