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

[Cornelia Huck <cohuck@redhat.com>]

On Tue, Jan 17 2023, Peter Maydell <peter.maydell@linaro.org> wrote:

> 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.

Ok, will change that.

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

I can certainly add "Future QEMU versions might change that behaviour."
We should be able to add compat handling for that.