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

[Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>]

On 10-07-2024 09:12 pm, Cornelia Huck wrote:
> On Mon, Jul 08 2024, Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com> wrote:
> 
>> Extend the 'mte' property for the virt machine to cover KVM as
>> well. For KVM, we don't allocate tag memory, but instead enable
>> the capability.
>>
>> 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.
>>
>> This patch is rework of commit b320e21c48ce64853904bea6631c0158cc2ef227
>> which broke TCG since it made the TCG -cpu max
>> report the presence of MTE to the guest even if the board hadn't
>> enabled MTE by wiring up the tag RAM. This meant that if the guest
>> then tried to use MTE QEMU would segfault accessing the
>> non-existent tag RAM.
> 
> So, the main difference to my original patch is that we don't end up
> with MTE in the max model if we didn't configure tag memory, but the
> rest of the behaviour stays the same?

Yes most of the patch is same. What I changed is to fix the code which 
was advertising MTE feature through PFR1 register for TCG based boot, 
irrespective of the tagged RAM allocated.

> 
>>
>> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
>> Signed-off-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
> 
> (...)
> 
>> +void kvm_arm_enable_mte(Object *cpuobj, Error **errp)
>> +{
>> +    static bool tried_to_enable;
>> +    static bool succeeded_to_enable;
>> +    Error *mte_migration_blocker = NULL;
>> +    int ret;
>> +
>> +    if (!tried_to_enable) {
>> +        /*
>> +         * MTE on KVM is enabled on a per-VM basis (and retrying doesn't make
>> +         * sense), and we only want a single migration blocker as well.
>> +         */
>> +        tried_to_enable = true;
>> +
>> +        ret = kvm_vm_enable_cap(kvm_state, KVM_CAP_ARM_MTE, 0);
>> +        if (ret) {
>> +            error_setg_errno(errp, -ret, "Failed to enable KVM_CAP_ARM_MTE");
>> +            return;
>> +        }
>> +
>> +        /* TODO: Migration is not supported with MTE enabled */
> 
> Do you have a plan for enabling migration in the future? From what I
> remember, pre-copy support should be doable within QEMU (with a similar
> approach to e.g. s390 skey), but post-copy would need a kernel API
> extension to support getting additional data while faulting in a page.
> 

No plan at the moment.

>> +        error_setg(&mte_migration_blocker,
>> +                   "Live migration disabled due to MTE enabled");
>> +        if (migrate_add_blocker(&mte_migration_blocker, errp)) {
>> +            error_free(mte_migration_blocker);
>> +            return;
>> +        }
>> +        succeeded_to_enable = true;
>> +    }
>> +    if (succeeded_to_enable) {
>> +        object_property_set_bool(cpuobj, "has_mte", true, NULL);
>> +    }
>> +}
> 

Thanks,
Ganapat