Kernel compile with builtroot and run it in qemu vm: fail to add MMCONFIG information, can’t access extended PCI configuration space under this bridge

[Julius Tischler]

Hello,

I found a nice tutorial on Linux kernel debugging with gdb + qemu +
vscode. (https://github.com/Rhydon1337/linux-kernel-debugging)

The big advantage would be, that I can use my familiar IDE vscode for
kernel debugging.

The first step in the tutorial is to compile the kernel using
builtroot and then run it in a qemu vm. I get stuck at part 9,
"running the qemu vm".

This is the command from the tutorial:
```
sudo ~/Downloads/qemu-7.1.0/bin/debug/native/x86_64-softmmu/qemu-system-x86_64
-enable-kvm -cpu host -s -kernel bzImage -m 2048 -hda rootfs.qcow2
-append "root=/dev/sda rw nokaslr" -net nic,model=virtio -net
user,hostfwd=tcp::5555-:22
```
In the output window it shows:
```
acpi PNP0A03:00: _OSC: OS supports [ASPM ClockPM Segments HPX-Type3]
acpi PNP0A03:00: fail to add MMCONFIG information, can’t access
extended PCI configuration space under this bridge.
```
Entire screenshot of output: https://i.imgur.com/D772zKA.png

What is even more confusing for me, I tried to adjust the command to
print the qemu console to my terminal, such that I can read the full
log.
```
sudo ~/Downloads/qemu-7.1.0/bin/debug/native/x86_64-softmmu/qemu-system-x86_64
-enable-kvm -cpu host -s -kernel bzImage -m 2048 -hda rootfs.qcow2
-append "root=/dev/sda rw nokaslr console=ttyS0" -nographic -serial
mon:stdio -net nic,model=virtio -net user,hostfwd=tcp::5555-:22
```
Then there is surprisingly no error message, but whatever command I
type into the console, when I hit enter, the vm doesn't react, nothing
happens.
Full log: https://pastebin.com/EFgnrAny

Host: Ubuntu Kubuntu 22.04.1 LTS; Linux 5.15.0-52; qemu 7.1.0; buildroot-2022.08
Guest: Linux Kernel downloaded by builtroot: 5.15.18

Any ideas, how I can figure out, what the problem is?

Best regards,
Julius Tischler