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

[Dr. David Alan Gilbert]

* 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.

Dave

> thanks
> -- PMM
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK