I noticed that after kernel commit abafbc55 'vfio-pci: Invalidate mmaps
and block MMIO access on disabled memory' vfio-pci via qemu on s390x
fails spectacularly, with errors in qemu like:

qemu-system-s390x: vfio_region_read(0001:00:00.0:region0+0x0, 4) failed: 
Input/output error

>From read to bar 0 originating out of hw/s390x/s390-pci-inst.c:zpci_read_bar().

So, I'm trying to figure out how to get vfio-pci happy again on s390x.  From
a bit of tracing, we seem to be triggering the new trap in
__vfio_pci_memory_enabled().  Sure enough, if I just force this function to
return 'true' as a test case, things work again.
The included patch attempts to enforce the setting, which restores everything
to working order but also triggers vfio_bar_restore() in the process....  So
this isn't the right answer, more of a proof-of-concept.

@Alex: Any guidance on what needs to happen to make qemu-s390x happy with this
recent kernel change?

@Nilkas/@Pierre: I wonder if this might be related to host device is_virtfn?
I note that my host device lspci output looks like:

0000:00:00.0 Ethernet controller: Mellanox Technologies MT27710 Family 
[ConnectX-4 Lx Virtual Function]

But the device is not marked as is_virtfn..  Otherwise, Alex's fix
from htps://lkml.org/lkml/2020/6/25/628 should cover the case.

Matthew Rosato (1):
  s390x/pci: Enforce PCI_COMMAND_MEMORY for vfio-pci

 hw/s390x/s390-pci-inst.c | 10 ++++++++++
 1 file changed, 10 insertions(+)


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