I was watching a video about the new X870E motherboards, and I have a question regarding PCIe lane sharing, particularly in a dual-boot setup with Linux.

Many X870E motherboards come with multiple Gen 5 M.2 slots (typically 1 or 2) that share bandwidth with the GPU. This means that if you install an SSD in one of these slots, the GPU will run at x8 instead of x16.

To clarify my question, let’s use the ASUS ROG Strix X870E motherboard as an example. Here’s the M.2 slot configuration (copied from the official specs):

There are 5 M.2 slots in total.
The M.2_2 and M.2_3 slots share bandwidth with the PCIe x16 (G5) slot, meaning if either of these slots is occupied by an SSD, the GPU will run at x8 instead of x16.
The other M.2 slots don’t affect GPU bandwidth.

Now, let's say I install Windows on an SSD in the M.2_1 slot (which doesn’t share lanes with the GPU), and Linux on an SSD in the M.2_2 slot (which does share lanes with the GPU).

When I boot into Windows, the SSD in M.2_2 (Linux) is not actively in use. Since the Linux SSD isn’t being accessed while I’m in Windows, will the GPU still run at x8, or will it run at x16?

My assumption is that the GPU will still run at x8 because the bandwidth sharing is a hardware limitation, and the motherboard likely cannot dynamically adjust PCIe lanes based on whether an SSD is being accessed or not. What do you think?