Re: [PATCH v2] hw/s390x: Fix a possible crash with passed-through virtio devices

[Thomas Huth <thuth@redhat.com>]

On 18/11/2025 15.25, Cornelia Huck wrote:
> On Tue, Nov 18 2025, Thomas Huth <thuth@redhat.com> wrote:
> 
>> On 18/11/2025 13.02, Halil Pasic wrote:
>>> On Tue, 18 Nov 2025 10:39:45 +0100
>>> Thomas Huth <thuth@redhat.com> wrote:
>>>
>>>> Consider the following nested setup: An L1 host uses some virtio device
>>>> (e.g. virtio-keyboard) for the L2 guest, and this L2 guest passes this
>>>> device through to the L3 guest. Since the L3 guest sees a virtio device,
>>>> it might send virtio notifications to the QEMU in L2 for that device.
...
>>> So I think it would really make sense to prevent passing through
>>> virtio-ccw devices with vfio-ccw.
>>
>> That could be a nice addition on top (in another patch), but we have to fix
>> handle_virtio_ccw_notify() anyway to avoid that the L3 guest can crash QEMU,
>> so it's certainly not a replacement for this patch, I think.
> 
> "Prevent crashing" is certainly the correct first step :)
> 
> I'm not sure where we would want prevent assignment of non-dasd
> devices. The kernel can't (because we're dealing with a subchannel
> driver.) I think maybe management software on top of QEMU?
> 
> The other direction would be supporting things like diag500, so we could
> pass through virtio-ccw devices as well. But I'm not really seeing a
> use case for it, or is there one?

It could be a very nice test setup to check vfio-ccw in CI pipelines (where 
you likely don't have access to a real DASD) ... but apart from that, I also 
don't see a real usecase for it.

  Thomas