ESXi 7 and GPU passthrough with Linux guests

I recently wasted spent several hours trying to get GPU passthrough to work in Linux guest on ESXi 7.0 with a AMD Radeon 8490. No joy. Tried it with a Windows guest, and within 20 minutes I had working video output.

Then I ‘disconnected’ the Windows drive from the Windows VM, and attached a virtual disk with a fully functional Linux install (Ubuntu 20.04 LTS). And nothing, no video output - AFAICT it wouldn’t even boot.

So what’s the secret sauce to getting a Linux VM to talk to my GPU?