pci passthrough on pv domain

[Adi Kriegisch <adi@xxxxxxxxxxxxxxx>]

Dear all,

I recently upgraded a small atom board server (C2550 cpu) from Debian
Buster to Bullseye, which corresponds to an upgrade from Xen 4.11 to
4.14 and noticed that pci passthrough does not work any more for pv
domains:

  | libxl: error: libxl_create.c:1107:libxl__domain_config_setdefault:
  |               passthrough not supported on this platform

I also tried attaching the pci device to the already running domU:

  | libxl: error: libxl_pci.c:1434:pci_add_dm_done:
  |               Domain 7:xc_assign_device failed: Operation not supported
  | libxl: error: libxl_pci.c:1721:device_pci_add_done:
  |               Domain 7:libxl__device_pci_add  failed for PCI device 0:3:0.0 
(rc -3)
  | libxl: error: libxl_device.c:1407:device_addrm_aocomplete:
  |               unable to add device

The device itself is claimed by the pciback driver in Dom0 and gets
listed by 'xl pci-assignable-list'.
The domU itself is just a PV machine booted via pvgrub and has
  | pci=['03:00.0',]
in the config.

When using Xen 4.11, both kernel versions available on Debian, 4.19 (old
stable) and 5.10, do allow pci pass through. 'xl dmesg' shows
  | (XEN) I/O virtualisation disabled
for both versions of Xen.

Can anyone help me fix pci passthrough?

Thanks!
-- Adi