Issues with IOMMU / gpu-passthrough on Proxmox

Using Proxmox hypervisor, trying to get a Windows10 guest OS to have direct GPU access. After following the instructions a few times, I found that there's probably a misconfiguration in the BIOS.

My setup:
MB: Asrock Rack X470D4U2-2T with bios v 2.20.1276
CPU: Ryzen7Pro 4750G
GPU: GeForce GTX 950

From the Proxmox Wiki: https://pve.proxmox.com/wiki/Pci_passthrough

Reboot, then run:
dmesg | grep -e DMAR -e IOMMU
There should be a line that looks like "DMAR: IOMMU enabled". If there is no output, something is wrong.

Well, there's no line that looks like that, so something is wrong.
A few bios settings that i've checked or enabled:
Advanced -> CPU Configuration -> SVM Mode: enabled
Advanced -> AMD CBS -> NBIO Common Options -> IOMMU enabled
Advanced -> AMD CBS -> NBIO Common Options -> DMA protection: enabled
Advanced -> AMD CBS -> NBIO Common Options -> DMAr Support: enabled

While on the subject, it seems that if I enable "DMA Protection" or "DMAr Support", i can no longer enter the bios -- there are 5 beeps during post, and pressing DEL key isn't detected, goes straight to OS.
(so i have taken the battery out to clear cmos a few times, to tinker with these settings).

I've been searching throughout the internet, trying all sorts of things, for pretty much all of the day. Any ideas what might be wrong? What else can I try?