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

[Dr. David Alan Gilbert]

* Cornelia Huck (cohuck@redhat.com) wrote:
> 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...

Yeh; at least make sure that any migration failure gives an obvious
message in the log.

Dave

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