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

[Cornelia Huck]

On Tue, Jan 17 2023, "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:

> * 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.
>> 
>> Should we write specifically that in a future QEMU version KVM
>> might change to defaulting to "on if available" when migration
>> support is implemented?
>
> Please make sure if you do something like that, that the failure
> is obious; 'on if available' gets messy for things like libvirt
> and higher level tools detecting features that are available and
> machines they can migrate to.

I guess we can just keep the door open but decline walking through it if
we fail to come up with a good solution...