permissions: 0.946
user-level: 0.926
mistranslation: 0.909
register: 0.907
debug: 0.903
graphic: 0.901
assembly: 0.889
kernel: 0.885
architecture: 0.881
semantic: 0.878
device: 0.876
files: 0.872
peripherals: 0.865
virtual: 0.855
arm: 0.854
performance: 0.850
x86: 0.850
hypervisor: 0.850
risc-v: 0.842
ppc: 0.818
VMM: 0.808
network: 0.799
PID: 0.799
KVM: 0.793
TCG: 0.778
i386: 0.778
boot: 0.778
vnc: 0.763
socket: 0.694

Can't bind PCI device behind a PCI bridge (No such device)
Description of problem:
Qemu fails to assign the device with :
```
qemu-system-x86_64: -device vfio-pci,host=3b:00.0: vfio 0000:3b:00.0: error getting device from group 72: No such device
Verify all devices in group 72 are bound to vfio-<bus> or pci-stub and not already in use
```

Looking at strace, we can see that the device is behind a PCI bridge:
```
lstat("/sys", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
lstat("/sys/bus", {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
lstat("/sys/bus/pci", {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
lstat("/sys/bus/pci/devices", {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
lstat("/sys/bus/pci/devices/0000:3b:00.0", {st_mode=S_IFLNK|0777, st_size=0, ...}) = 0
readlink("/sys/bus/pci/devices/0000:3b:00.0", "../../../devices/pci0000:3a/0000"..., 4095) = 53
lstat("/sys/devices", {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
lstat("/sys/devices/pci0000:3a", {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
lstat("/sys/devices/pci0000:3a/0000:3a:02.0", {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
lstat("/sys/devices/pci0000:3a/0000:3a:02.0/0000:3b:00.0", {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
lstat("/sys/devices/pci0000:3a/0000:3a:02.0/0000:3b:00.0/subsystem", {st_mode=S_IFLNK|0777, st_size=0, ...}) = 0
readlink("/sys/devices/pci0000:3a/0000:3a:02.0/0000:3b:00.0/subsystem", "../../../../bus/pci", 4095) = 19
lstat("/sys/bus", {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
lstat("/sys/bus/pci", {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
ioctl(14, VFIO_GROUP_GET_DEVICE_FD, 0x56267b3b1320) = -1 ENODEV (No such device)
```

The issue is that the PCI bridge `0000:3a:02.0`, is used by "pcieport" kernel driver and not "vfio-pci".
After manually unbinding the PCI bridge from it's driver and binding it to vfio-pci qemu successfully attaches it to the VM.

I saw online that qemu is suposed to automaticly unbind devices from the host, make them available to the VM and restore them to their previous state once the VM is shutdown. 
This is not happening here.
Steps to reproduce:
1. Have a PCI device behind a PCI bridge
2. Launch a VM with the PCI device attached
3. Observe similar error messages
Additional information:
After reading [kernel vfio doc](https://www.kernel.org/doc/html/latest/driver-api/vfio.html#vfio-usage-example), I can see that `ls -l /sys/bus/pci/devices/0000:3b:00.0/iommu_group/devices` was supposed to list the PCI bridge, but it is not the case for me.

I could only notice the presence of the bridge by looking in the `/sys/bus/pci/devices/0000:3b:00.0` symlink.

Maybe qemu misses it because of that ?