Since long time ago, to enable the SR-IOV VF pci passthrough function, I'm always adding "amd_iommu=on" into kernel cmdline on AMD system. 
But recently I found even I do not do this action, IOMMU is still enabled by kernel on AMD system. 
After searching, I found there is no such setting any more, refer to https://github.com/torvalds/linux/blob/master/Documentation/admin-guide/kernel-parameters.txt#L286

There are only 3 possible value as below:
amd_iommu= fullflush

Could anyone can help to confirm the changes?  Thank you! 
And another question, it is said that the "iommu=pt" option is to improves IO performance for devices in the host, it is not a must for VF PCI passthrough, right?
I'm not sure about the user cases. 

1. Reference about adding "amd_iommu=on", and it may be outdated:  http://dpdk-guide.gitlab.io/dpdk-guide/setup/iommu.html
2. On AMD system without adding "amd_iommu=on"  in the kernel cmdline, the iommu is enabled:
# cat /proc/cmdline
BOOT_IMAGE=(hd0,msdos1)/vmlinuz-4.18.0-193.el8.x86_64 root=/dev/mapper/rhel_hp--dl385g10--16-root ro crashkernel=auto resume=/dev/mapper/rhel_hp--dl385g10--16-swap rd.lvm.lv=rhel_hp-dl385g10-16/root rd.lvm.lv=rhel_hp-dl385g10-16/swap console=ttyS0,115200n81

# dmesg | grep -i iommu
[    3.712029] iommu: Default domain type: Passthrough
[    6.736019] pci 0000:00:00.2: IOMMU performance counters supported
[    6.780040] pci 0000:e0:00.2: IOMMU performance counters supported
[    6.786740] pci 0000:00:01.0: Adding to iommu group 0
[    6.791876] pci 0000:00:01.1: Adding to iommu group 1
[    6.797015] pci 0000:00:01.2: Adding to iommu group 2
[    6.802150] pci 0000:00:01.4: Adding to iommu group 3
[    7.866463] pci 0000:e0:00.2: Found IOMMU cap 0x40
[    7.920222] perf/amd_iommu: Detected AMD IOMMU #0 (2 banks, 4 counters/bank).
[    7.927428] perf/amd_iommu: Detected AMD IOMMU #1 (2 banks, 4 counters/bank)

Best Regards,
Yalan Zhang
IRC: yalzhang