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

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 22 Sep 2017 14:40:31 +0200
Christian Borntraeger <borntraeger@de.ibm.com> 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. 

Yes, that will probably still need quite a bit of work before we are
there.

> 
> And for emulated devices under KVM you still need the kernel support - otherwise migration is really broken for
> the nimm/dimm values.

Agreed, you need this for any pci device.

I have not yet reviewed this (or tested under tcg). I'd like to hear
David's opinion, though.