Re: [PATCH v2 1/1] virtio-ccw: auto-manage VIRTIO_F_IOMMU_PLATFORM if PV

[Halil Pasic <pasic@linux.ibm.com>]

On Tue, 9 Jun 2020 18:05:59 +0200
Cornelia Huck <cohuck@redhat.com> wrote:

> > 
> > do we really have that many incompatible devices?  
> 
> Which devices are compatible in the end? It seems the only ones that
> are known to be working are virtio-ccw devices with IOMMU_PLATFORM on.
> virtio-pci devices and non-virtio ccw (vfio-ccw, 3270) seem to be out,
> as far as I understand it. What about non-ccw? PCI in general, vfio-ap?

AFAIU the situation is somewhat complicated. Only virtio devices have
the notion of indicating and the duty to indicate access restrictions.
We as hypervisor need to prevent the inconsistency where:
* the VM is prot virt
* the guest can detect that it is running with prot virt using the UV
  call interface
* and the virtio devices emulated by us (QEMU) lie that memory access
  by the device is not restricted (F_ACCESS_PLATFORM not offered).

It is unfortunate that the consequences are this severe, but it is the
responsibility to drive the devices accordingly if prot virt is
detected. If the guest does this, then s270 and vfio-ccw should work. The
problem is, that currently Linux is verifiedly doing the thing only for
virtio-ccw.

Regards,
Halil