Confused about WSLg + Xvfb; can Xvfb really talk to the GPU?

[caffeine-storm]

Hello all,

Hopefully I'm just confused about the layering of things or something but I'm seeing some unexpected behaviour.

I expected Xvfb to use a software renderer (like llvmpipe) but it seems to be using the d3d12 driver?

For me, since Xvfb is not using the software renderer, some automated testing in my OpenGL project isn't consistent between WSL and 'real' Ubuntu instances. I can work around it for now with $LIBGL_ALWAYS_SOFTWARE but I want to better understand why this happens. Is it a bug in WSLg?

---

My methodology:

When I run glxinfo -B in a fresh WSL terminal I see, as expected, that the D3D12 'device' is used:

$> glxinfo -B
glxinfo -B
name of display: :0
display: :0  screen: 0
direct rendering: Yes
Extended renderer info (GLX_MESA_query_renderer):
    Vendor: Microsoft Corporation (0xffffffff)
    Device: D3D12 (NVIDIA GeForce RTX 3080 Ti) (0xffffffff)
    Version: 24.2.8
    Accelerated: yes
...

When I run LIBGL_ALWAYS_SOFTWARE glxinfo -B I see, as expected, that the llvmpipe 'device' is used:

$> LIBGL_ALWAYS_SOFTWARE=true glxinfo -B
name of display: :0
display: :0  screen: 0
direct rendering: Yes
Extended renderer info (GLX_MESA_query_renderer):
    Vendor: Mesa (0xffffffff)
    Device: llvmpipe (LLVM 19.1.1, 256 bits) (0xffffffff)
    Version: 24.2.8
    Accelerated: no
...

When I run xvfb-run glxinfo -B I see

$> xvfb-run glxinfo -B
name of display: :99
display: :99  screen: 0
direct rendering: Yes
Extended renderer info (GLX_MESA_query_renderer):
    Vendor: Microsoft Corporation (0xffffffff)
    Device: D3D12 (NVIDIA GeForce RTX 3080 Ti) (0xffffffff)
    Version: 24.2.8
    Accelerated: yes
...

which, to me, is weird; I wouldn't expect the Xvfb server to expose what looks like "real hardware" to its X clients.