[PATCH v7 10/17] KVM: arm64: introduce new KVM ITS device

[Auger Eric]

Hi Peter,

On 04/07/2016 16:32, Peter Maydell wrote:
> On 4 July 2016 at 15:27, Auger Eric <eric.auger at redhat.com> wrote:
>> Andre,
>>
>> On 04/07/2016 16:05, Andre Przywara wrote:
>>> Hi,
>>>
>>> On 04/07/16 10:00, Auger Eric wrote:
>>>> From a QEMU integration point of view this means the init sequence used
>>>> for KVM GIC interrupt controllers cannot be reused for ITS and more
>>>> importantly this is not straightforward to have the proper sequence
>>>> ordering (hence the previously reported case).
>>>
>>> I am confused, can you please elaborate what the problem is?
>>> Or alternatively sketch what you ideally would the ITS init sequence to
>>> look like? I am totally open to any changes, just need to know what
>>> you/QEMU needs.
>>
>> in QEMU the address setting is done on a so-called qemu
>> "machine_init_done_notifier", ie. a callback that is registered at ITS
>> device init, to be called once the virt machine code has executed. This
>> callback calls  kvm_device_ioctl(kd->dev_fd, KVM_SET_DEVICE_ATTR, attr);
>>
>> In case the userspace needs to explicitly "init" the ITS (actually ~
>> map_resources) this must happen after the KVM_SET_DEVICE_ATTR. So you
>> also must register a callback in the same way. However there is a
>> framework existing to register kvm device addresses but this does not
>> exist to set other attributes than device addresses.
>>
>> This is feasible I think but this does not fit qemu nicely. So can't the
>> map_resources happen implicitly on the first VCPU run?
> 
> I'm not clear what you think the problem here for QEMU is.
> We definitely want the API for the kernel to be:
>  create device
>  set attributes
>  explicitly complete init of the device
>  [attribute setting after this is illegal]
>  run CPUs
> 
> so I'm not sure why QEMU would care if the kernel does things at
> "final init" rather than "run CPUs".
> 
> This is how the GICv3 init works and how the ITS should work too;
The GICv3 explicit does not do the same as the ITS init.
GICv3 init does not map the resources (KVM iodevice registration). This
is done at 1st VCPU run.
ITS init does map the resources. If we call the ITS init at the same
place as we call the GICv3 init, in the realization function, the region
mapping is not yet done so you will map resources at undefined location.

I am definitively not opposed to call the ITS init function explicitly
from user side but this must happen after the KVM_SET_DEVICE_ATTR. So
another machine_init_done function must be registered and the notifier
must be called AFTER the notifier that calls the KVM_SET_DEVICE_ATTR
ioctl. However you cannot easily master the machine init done notifier
registration order because in target-arm/kvm.c there is a single
notifier that calls all the KVM_SET_DEVICE_ATTR for all the KVM devices
(kvm_arm_machine_init_done). So it is not possible to register the ITS
init notifier before the "kvm_arm_set_device_addr" notifier.

So my understanding is one must do things outside of the existing framework?

Hope this clarifies

Thanks

Eric


> we don't want to extend the GICv2 mistake of "no explicit complete
> init" to anything else, because then you end up with ad-hoc
> "do this when we first run the vCPU; oh, but also do it if
> userspace tries to write a register content; and also if...".
> 
> thanks
> -- PMM
>