Alright, here is the play by play on my process of getting 3090 SLI or NVLink to work in Red Dead Redemption 2 (RDR2) Vulkan mode (for those who may have the same issues I experienced).
***Make sure SLI is enabled in the Nvidia Control Panel***
1) First issue was getting RDR2 to run at all with SLI Enabled in the NVidia Control Panel. Without this change, my SLI would just result in black screens with sound for both the benchmark and the game itself.
[a] Solution: changing transferQueuesEnabled from 'false' to 'true' in the system.xml file located in C:\Users\*****YOUR USER NAME ON YOUR PC*****\OneDrive\Documents\Rockstar Games\Red Dead Redemption 2\Settings\system.xml
Make sure to edit in notepad and not to allow any formatting changes to be made or added. Also, the file won't show the lines that you need to edit unless you open it in something like notepad or txt edit (xml viewer hides the lines for some reason...).
2) The next issue was getting the SLI to actually enable for the 3090s. This I suspect has to do with some kind of conflict between programs accessing and changing the nvdrsdb0.bin and nvdrsdb1.bin files independently of each other for some reason.
[a] Download and install nvidiaProfileInspector.
Change the following settings on Red Dead Redemption 2 profile:
- In section 5 - Common: change Power Management Mode to Prefer Maximum Performance
- [optional] Antialiasing - SLI AA to Enabled
- Change the 'Number of GPUs to use on SLI Rendering Mode' to '0x00000002 SLI_GPU_COUNT_TWO'
- Change the 'NVIDIA predefined number of GPUs to use on SLI rendering mode' to '0x00000002 SLI_PREDEFINED_GPU_COUNT_TWO'
- Change the 'NVIDIA predefined SLI mode' to ''0x00000003 SLI_PREDEFINED_MODE_FORCE_AFR2'
- Change the 'SLI rendering mode' to '0x00000003 SLI_RENDERING_MODE_FORCE_AFR2'
[c] After applying the above changes immediately change the nvdrsdb0.bin and nvdrsdb1.bin files (Location: C:\ProgramData\NVIDIA Corporation\Drs) to 'read only'
- Note: You will have to go to Appearance and Personalization in the Folder Options in Control Panel and select 'Show Hidden Files, Folders, and Drives' to access these easily.
That is what worked for me at least. The video I found for part two above was helpful only for the above info, so I didn't bother posting it. Some of the other info in the video is misleading (like using GTA V profile instead of RDR2 profile) and didn't make any difference for me. Hope this helps someone. I think most of the info above in step one was in this thread, but not sure; I was all over the place yesterday to find a combination that would work for me.