[Bug 1869006] Re: PCIe cards passthrough to TCG guest works on 2GB of guest memory but fails on 4GB (vfio_dma_map invalid arg)

[Peter Maydell]

My (limited) understanding of PCI was that the board and the PCI
controller emulation define what the windows are where PCI BARs can
live, and that PCI cards go there. (This certainly works for emulated
PCI cards.) It's not clear to me why the host system imposes
restrictions on the memory layout of the VM...

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Title:
  PCIe cards passthrough to TCG guest works on 2GB of guest memory but
  fails on 4GB (vfio_dma_map invalid arg)

Status in QEMU:
  New

Bug description:
  During one meeting coworker asked "did someone tried to passthrough
  PCIe card to other arch guest?" and I decided to check it.

  Plugged SATA and USB3 controllers into spare slots on mainboard and
  started playing. On 1GB VM instance it worked (both cold- and hot-
  plugged). On 4GB one it did not:

  Błąd podczas uruchamiania domeny: internal error: process exited while 
connecting to monitor: 2020-03-25T13:43:39.107524Z qemu-system-aarch64: -device 
vfio-pci,host=0000:29:00.0,id=hostdev0,bus=pci.3,addr=0x0: VFIO_MAP_DMA: -22
  2020-03-25T13:43:39.107560Z qemu-system-aarch64: -device 
vfio-pci,host=0000:29:00.0,id=hostdev0,bus=pci.3,addr=0x0: vfio 0000:29:00.0: 
failed to setup container for group 28: memory listener initialization failed: 
Region mach-virt.ram: vfio_dma_map(0x563169753c80, 0x40000000, 0x100000000, 
0x7fb2a3e00000) = -22 (Invalid argument)

  Traceback (most recent call last):
    File "/usr/share/virt-manager/virtManager/asyncjob.py", line 75, in 
cb_wrapper
      callback(asyncjob, *args, **kwargs)
    File "/usr/share/virt-manager/virtManager/asyncjob.py", line 111, in tmpcb
      callback(*args, **kwargs)
    File "/usr/share/virt-manager/virtManager/object/libvirtobject.py", line 
66, in newfn
      ret = fn(self, *args, **kwargs)
    File "/usr/share/virt-manager/virtManager/object/domain.py", line 1279, in 
startup
      self._backend.create()
    File "/usr/lib64/python3.8/site-packages/libvirt.py", line 1234, in create
      if ret == -1: raise libvirtError ('virDomainCreate() failed', dom=self)
  libvirt.libvirtError: internal error: process exited while connecting to 
monitor: 2020-03-25T13:43:39.107524Z qemu-system-aarch64: -device 
vfio-pci,host=0000:29:00.0,id=hostdev0,bus=pci.3,addr=0x0: VFIO_MAP_DMA: -22
  2020-03-25T13:43:39.107560Z qemu-system-aarch64: -device 
vfio-pci,host=0000:29:00.0,id=hostdev0,bus=pci.3,addr=0x0: vfio 0000:29:00.0: 
failed to setup container for group 28: memory listener initialization failed: 
Region mach-virt.ram: vfio_dma_map(0x563169753c80, 0x40000000, 0x100000000, 
0x7fb2a3e00000) = -22 (Invalid argument)

  
  I played with memory and 3054 MB is maximum value possible to boot VM with 
coldplugged host PCIe cards.

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