Enable in BIOS

IOMMU is under Advanced > AMD CBS > NBIO Common Options.

Enable in GRUB

vim /etc/default/grub

You will see the line with GRUB_CMDLINE_LINUX_DEFAULT="quiet", all you need to do is add intel_iommu=on or amd_iommu=on depending on your system.

# Should look like this
GRUB_CMDLINE_LINUX_DEFAULT="quiet intel_iommu=on"

Update grub and reboot system

update-grub
shutdown -r now

Now check to make sure everything is enabled.

dmesg | grep -e DMAR -e IOMMU
dmesg | grep 'remapping'

Should look something like the following

root@pve1:~# dmesg | grep -e DMAR -e IOMMU
[    0.550475] pci 0000:c0:00.2: AMD-Vi: IOMMU performance counters supported
[    0.553245] pci 0000:80:00.2: AMD-Vi: IOMMU performance counters supported
[    0.556894] pci 0000:40:00.2: AMD-Vi: IOMMU performance counters supported
[    0.561552] pci 0000:00:00.2: AMD-Vi: IOMMU performance counters supported
[    0.568226] perf/amd_iommu: Detected AMD IOMMU #0 (2 banks, 4 counters/bank).
[    0.568231] perf/amd_iommu: Detected AMD IOMMU #1 (2 banks, 4 counters/bank).
[    0.568238] perf/amd_iommu: Detected AMD IOMMU #2 (2 banks, 4 counters/bank).
[    0.568243] perf/amd_iommu: Detected AMD IOMMU #3 (2 banks, 4 counters/bank).
root@pve1:~# dmesg | grep 'remapping'
[    0.566174] AMD-Vi: Interrupt remapping enabled
root@pve1:~#