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

[Cornelia Huck]

On Wed, Jun 29 2022, Eric Auger <eauger@redhat.com> wrote:

> 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

With OnOffAuto, I currently have:

valid for tcg: cpu.mte=on, tag_memory=on (result: mte on)
               cpu.mte=off, tag_memory either on or off (result: mte off)
               cpu.mte unspecified, tag_memory either on or off (result:
               mte==tag_memory)
valid for kvm: tag_memory always off
               cpu.mte=off (result: mte off)
               cpu.mte=on if mte supported in kvm (result: mte on)
               cpu.mte unspecified (result: mte on if kvm supports it;
               this I can flip)
all other combinations: error