Re: [Qemu-devel] [PATCH v1 0/5][RFC] Refactoring of AIS support

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

On 30/10/2017 13:44, Christian Borntraeger wrote:
> 
> 
> On 10/30/2017 01:42 PM, Cornelia Huck wrote:
>> On Mon, 30 Oct 2017 09:28:09 +0100
>> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
>>
>>> Now I thought about that for a while and I start to think that we cannot implement ais
>>> in QEMU and cover all cases.
>>> One aspect was certainly passthrough (like you handled in patch 4).
>>> Another aspect is that some interrupts might be injected from the kernel - even for
>>> emulated devices. e.g. virtio-pci together with vhost-net, will inject interrupts via
>>> the set_irq callback. I think disabling irqfd for these cases is not a good idea.
>>
>> Is there still a fallback for irqfd emulation?
> 
> it might disable dataplane or other things. (it once did). So I think we should not
> go down this path.
> 
>>
>>>
>>> So what about adding a new KVM capability (for 4.14), fixup the other things in
>>> QEMU and then bind it to the new capability?
>>
>> For 4.15, surely?
>>
>> Probably the only way we can make this work correctly...
>>

I may have not understand.

Why do we need a new capability, we already have the KVM_CAP_S390_AIS 
capability?
The PCI device has a netdev property pointing to a netdev, if this 
netdev sets the vhost property, can't we test this to know if we can 
realize this device or not?

Using virtio-pci instead of virtio-ccw is not the first choice for S390. 
The use case I see for S390 using virtio-pci is as a fallback in case 
for the migration of a PCI device the target host does not support AIS 
or do not have VFIO device and one do not want to modify the guest.

Pierre


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