Re: [PATCH RFC 0/2] arm: enable MTE for QEMU + kvm

[Eric Auger]

Hi Connie,

On 6/13/22 18:02, Cornelia Huck wrote:
> On Fri, Jun 10 2022, Eric Auger <eauger@redhat.com> wrote:
> 
>> Hi Connie,
>>
>> On 5/12/22 15:11, Cornelia Huck wrote:
>>> This series enables MTE for kvm guests, if the kernel supports it.
>>> Lightly tested while running under the simulator (the arm64/mte/
>>> kselftests pass... if you wait patiently :)
>>>
>>> A new cpu property "mte" (defaulting to on if possible) is introduced;
>>> for tcg, you still need to enable mte at the machine as well.
>> isn't the property set to off by default when kvm is enabled (because of
>> the migration blocker).
> 
> Oh, I had changed that around several times, and it seems I ended up
> being confused when I wrote this cover letter... I wonder what the best
> state would be (assuming that I don't manage to implement it soonish,
> but it seems we still would need kernel changes as by the discussion in
> that other patch series.)
Having mte=off by default along with KVM, until the migration gets
supported, looks OK to me. Does it prevent you from having it set to
another value by default with TCG (depending on the virt machine
tag_memory option)?

                tag_memory=on   tag_memory=off
KVM CPU mte=off invalid         mte=off
KVM CPU mte=on  invalid         mte=on
TCG CPU mte=off invalid         mte=off
TCG CPU mte=on  mte=on          invalid

default value:
KVM mte = off until migration gets supported
TCG mte = machine.tag_memory

Thanks

Eric

> 
>>
>> Eric
>>>
>>> I've hacked up some very basic qtests; not entirely sure if I'm going
>>> about it the right way.
>>>
>>> Some things to look out for:
>>> - Migration is not (yet) supported. I added a migration blocker if we
>>>   enable mte in the kvm case. AFAIK, there isn't any hardware available
>>>   yet that allows mte + kvm to be used (I think the latest Gravitons
>>>   implement mte, but no bare metal instances seem to be available), so
>>>   that should not have any impact on real world usage.
>>> - I'm not at all sure about the interaction between the virt machine 'mte'
>>>   prop and the cpu 'mte' prop. To keep things working with tcg as before,
>>>   a not-specified mte for the cpu should simply give us a guest without
>>>   mte if it wasn't specified for the machine. However, mte on the cpu
>>>   without mte on the machine should probably generate an error, but I'm not
>>>   sure how to detect that without breaking the silent downgrade to preserve
>>>   existing behaviour.
>>> - As I'm still new to arm, please don't assume that I know what I'm doing :)
>>>
>>>
>>> Cornelia Huck (2):
>>>   arm/kvm: enable MTE if available
>>>   qtests/arm: add some mte tests
>>>
>>>  target/arm/cpu.c               | 18 +++-----
>>>  target/arm/cpu.h               |  4 ++
>>>  target/arm/cpu64.c             | 78 ++++++++++++++++++++++++++++++++++
>>>  target/arm/kvm64.c             |  5 +++
>>>  target/arm/kvm_arm.h           | 12 ++++++
>>>  target/arm/monitor.c           |  1 +
>>>  tests/qtest/arm-cpu-features.c | 31 ++++++++++++++
>>>  7 files changed, 137 insertions(+), 12 deletions(-)
>>>
>