On Tue, Feb 28 2023, Andrea Bolognani <abologna@xxxxxxxxxx> wrote: > On Tue, Feb 28, 2023 at 04:02:15PM +0100, Cornelia Huck 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 | 110 +++++++++++++++++++++++++++++++ >> target/arm/internals.h | 1 + >> target/arm/kvm.c | 29 ++++++++ >> target/arm/kvm64.c | 5 ++ >> target/arm/kvm_arm.h | 19 ++++++ >> target/arm/monitor.c | 1 + >> 10 files changed, 194 insertions(+), 13 deletions(-) > > I've given a quick look with libvirt integration in mind, and > everything seem fine. > > Specifically, MTE is advertised in the output of qom-list-properties > both for max-arm-cpu and the latest virt-X.Y-machine, which means > that libvirt can easily and reliably figure out whether MTE support > is available. Great, thanks for having a look! > >> +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. > > Is it okay to use -machine virt,mte=on unconditionally for both KVM > and TCG guests when MTE support is requested, or will that not work > for the former? QEMU will error out if you try this with KVM (basically, same behaviour as before.) Is that a problem for libvirt, or merely a bit inconvinient? > >> +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 if and only if tag memory is available; >> + i.e. it preserves the behaviour prior to the introduction of the feature. >> + >> +* When KVM is used, MTE will default to off, so that migration will not >> + unintentionally be blocked. This might change in a future QEMU version. > > If and when this changes, we should ensure that the new default > behavior doesn't affect existing machine types, otherwise we will > break guest ABI for existing VMs. Nod, such a change would need proper compat handling. It's not quite clear yet if we'll ever flip it, though.
