Re: [Qemu-devel] [PATCH/RFC 2/3] s390x/ais: enable ais when migration is available

[Pierre Morel <pmorel@linux.vnet.ibm.com>]

On 22/09/2017 14:40, Christian Borntraeger wrote:
> 
> 
> On 09/22/2017 02:13 PM, Pierre Morel wrote:
>> On 22/09/2017 10:38, Christian Borntraeger wrote:
>>> Instead of unconditionally enabling the KVM AIS capability
>>> in the kvm arch init function, do this in the flic realize function
>>> when we know if migration is available. This requires to initialize
>>> flic before the CPUs.
>>
>> I am not sure to agree.
>>
>> AIS facility is used for PCI (currently only PCI)
>> We want to support PCI emulation and PCI VFIO
>>
>>
>> Not having AIS support in the host kernel or not supporting AISM in the host kernel does not affect the emulation.
>> Neither virtio-pci nor TCG.
>> The only devices, (currently), which can not work without AIS and is not migratable without AISM are PCI VFIO devices.
> 
> This patch enable the conditional enablement facility for the KVM host mask. The cpu model enablement is done
> differently for KVM and TCG anyway.
> Right now AIS is only enabled for KVM. For TCG AIS is not implemented at all and disabled. So for whenever this
> is fixed in TCG it can be handled then.
> 
> And for emulated devices under KVM you still need the kernel support - otherwise migration is really broken for
> the nimm/dimm values.
> 

Yes, that is right.
I do not pretend that it is working directly.

To support AIS emulation we have some work to do there (the migration), 
in kvm_s390_inject_airq() and other functions using ais_supported, to 
use emulated values.

Don't you think that we could use emulation instead of faulting when the 
kernel can not handle values needed for pass-through?

We can fault at the moment a VFIO device is realized.
Migration will be stopped on realizing VFIO PCI devices instead of flic.

I can miss something, is there a reason not to do so?


-- 
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany