On Wed, 11 Jan 2023 at 16:13, Cornelia Huck <cohuck@xxxxxxxxxx> 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@xxxxxxxxxx> > --- > 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? thanks -- PMM
