[question] Manual execution

[sieskei]

Hello, I have an interesting case. I have an Asus M16 2021 laptop and I'm using an eGPU (RX 6800 XT) through an Oculink x4 Gen 4.0 connection. It's connected via the NVMe port and works great, but without resizable BAR (re-bar) support.

I managed to enable resizable BAR support using setup_var.efi, but it only works for the built-in GPU (dGPU, 3060). I even disabled the dGPU and iGPU through the BIOS, but it still doesn't work.

An interesting fact is that in Linux, I can boot with pci=realloc and set bar2 to 16GB. I can even dynamically resize it at runtime using resource2_resize.

So, my thought is the following: Is it possible to create this an EFI tool that can be invoked manually before booting into Windows?

It would be great for people like me who don't have the option to flash the entire BIOS.

Thank you in advance!
Miro.

[xCuri0]

@sieskei this module works by hooking PreprocessController which gets called by the PciBus module during enumeration. unless you can figure out how to re-enumerate PCI I don't think so.

using kexec to boot windows from linux is possible and has been done before.

see https://github.com/osresearch/safeboot-loader. you will need to setup a minimal kernel + initrd which does the bar resizing and then boots into windows

[xCuri0]

@sieskei dxe should work if the BIOS doesn't have anything preventing it from being modified

[bannhead]

@xCuri0 thank you!

Does the fact that the BIOS does not support ReBAR on that port mean that even if DXE is added, it still won't work?

The kexec option seems interesting, and I would like to try it. However, I'm having trouble finding much available information suitable for my level of experience and knowledge in Linux.

Hi @sieskei, were you able to put together a working bootloader to start the notebook with nvme activated rebar in Windows? I'm in the same situation as you, with a Dell laptop that can't change the bios

[xCuri0]

@bannhead have you tried setting up safeboot-loader ?

you just need to set it up and then use sysfs (resource#_resize) to resize bar before chainloading windows. you may need to edit DSDT too if your laptop doesn't have 4g decoding

[sieskei]

Hello, I haven't tried it yet because I spent a long time attempting to flash the BIOS, but it seems that it won't work without a programmer.

This bootloader looks interesting. Is it still supported? In the description, it mentions that it uses kernel version 5. Are the features related to ReBar available in versions 6 and above?

[xCuri0]

@sieskei afaik kernel version 5 has them but doesn't expose them through sysfs. But it shouldn't be too hard to write a simple kernel driver that resizes it