libvirt users - Jun 2020 - Do we need "amd_iommu=on" for AMD system anymore?

Hi,

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
off
force_isolation

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.

[Reference]
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 Reference about adding "amd_iommu=on", and it may be outdated: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

On Tue, Jun 30, 2020 at 11:33:10AM +0800, Yalan Zhang
wrote:
> Hi,
> 
> 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.
I don't know history of amd_iommu=on, but currently libvirt's official
recommendation for AMD hosts, via virt-host-validate is

   iommu=pt  iommu=1

Regards,
Daniel
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