Re: [PATCH v6 1/2] arm/kvm: add support for MTE

[Cornelia Huck <cohuck@redhat.com>]

On Fri, Mar 03 2023, Peter Maydell <peter.maydell@linaro.org> wrote:

> On Tue, 28 Feb 2023 at 15:02, 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               | 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(-)
>
>
>
>> +static inline bool arm_machine_has_tag_memory(void)
>> +{
>> +#ifndef CONFIG_USER_ONLY
>> +    Object *obj = object_dynamic_cast(qdev_get_machine(), TYPE_VIRT_MACHINE);
>> +
>> +    /* so far, only the virt machine has support for tag memory */
>> +    if (obj) {
>> +        VirtMachineState *vms = VIRT_MACHINE(obj);
>> +
>> +        return vms->mte;
>> +    }
>
> Code inside target/arm shouldn't be fishing around inside
> the details of the board model like this. For TCG I think that
> at this point (i.e. at realize) you should be able to tell if
> the board has set up tag memory, because it will have set
> cpu->tag_memory to non-NULL.

I agree that we shouldn't need to poke into the machine innards here,
but I found that it was actually too early to check for cpu->tag_memory --
details have unfortunately been flushed out of my cache already, can try
to repopulate.