I had a dual boot Windows/Fedora machine, and I wanted to be able to run the Fedora install as a VM inside WSL. To do this, I needed the physical disk accessible from the WSL kernel. Since the WSL kernel has no access to physical hardware like that, I wanted to try using a nbd server on Windows and a nbd client on Linux.
Does the default WSL kernel support nbd?
$ zgrep NBD /proc/config.gz # CONFIG_BLK_DEV_NBD is not set
The answer is no, it does not. You can, however, use your own kernel.
Use git to check out this repo: https://github.com/microsoft/WSL2-Linux-Kernel
I'm using the linux-msft-4.19.128 tag, but it looks like there's a 5.4.x branch as well. That repo has a file README-Microsoft.WSL2 with details on what packages are needed. They have their default config in Microsoft/config-wsl, which I turned on NBD support.
I had tried compiling 4.19.121 on a system with gcc 10.2.1, and that got me a kernel that wouldn't boot. Looks like 4.19.124 has some fixes for gcc 10. Building with gcc 9.3.0 worked.
Building the kernel
make -j4 KCONFIG_CONFIG=Microsoft/config-wsl
This will produce the kernel (we need the vmlinux file) and modules. You can change the 4 to match the number of cores you have.
Installing the kernel
I put the kernel on my Windows partition:
mkdir /mnt/c/Users/myuser/kernel cp vmlinux /mnt/c/Users/myuser/kernel/vmlinux-4.19.128
And then I set the wsl config file to use it:
The kernel path does need double \\
Running the kernel
I shutdown my WSL VMs with "wsl --shutdown", and then started them with the new kernel.
C:\Users\myuser>wsl cat /proc/version Linux version 4.19.128-microsoft-standard+ (myuser@wsl) (gcc version 9.3.0 (Ubuntu 9.3.0-10ubuntu2)) #3 SMP Fri Aug 28 01:10:03 CDT 2020
And now it has nbd support!
$ zgrep NBD /proc/config.gz CONFIG_BLK_DEV_NBD=y
I had hoped to use cygwin to run nbd-server on Windows, but cygwin's pread does not work against physical drives, only files.
If you make your own kernel, remember to keep it up to date.