Which one decides the IOMMU Groups? CPU, Motherbord, BIOS, Operating system?

[TariCaesar]

I'm building a NAS on KVM host. And I want to passthrough the SATA controller to the guest.

But after I enable the IOMMU, I found the SATA controller, Pciex4 M.2 connecting to the CPU directly and some other peripherals are divided into the same IOMMU group like this:

 

In my thought the M.2 and sata controller use different lanes. M.2 connects to CPU directly, SATA connects to the chipset. Why they are placed in the same group?

If I buy a new motherboard, will things be different?

 

My hardware information:

CPU: AMD-Ryzen 5650ge

Motherboard: ASRock A520M-ITX/ac

[manikyath]

from what i understand of it, it's sort of a 'combination of everything' deal, the motherboard being the most important part ofcourse, because that decides how things are connected.

i havent checked where my onboard sata ports sit on my unraid box, but i'd hazard a guess that trying to pass those trough will break other things.

for passing trough you always best just go for add-in cards, or for parts of the motherboard that are completely separate chipsets wired to pcie (like for example an extra usb chipset may exist)

[leadeater]

The default is the motherboard, BIOS and also actual PCIe layout. Updating to latest Kernel can remedy some weird IOMMU groupings however on most consumer/gaming motherboards IOMMU grouping is on no concern at all and can be done really badly. This is when you have to resort to manual overrides and creating your own groups. I believe this is not hard to do however I've never actually needed to do it myself.

[Jarsky]

Its generally determined by the chipset/motherboard and PCIe (electrical connectivity) itself. 

You could try an ACS Override patch on the kernel, but you may or may not have stability issues. 

You could also assign all the devices in the group to the VFIO-PCI driver, and just pass the SATA controller through, assuming you arent trying to passthrough anything else in the group elsewhere. 

 

Heres my ASRock X570 Taichi

 

 

 

 

[TariCaesar]

Thanks for replying. After struggling on the messy motherboard for two days I decided to use OMV as the virtual host os while acting as a NAS also. Thus the system runs in phyisical environment so I can use gpu 2D acceleration and sata controller.

Maybe running nas on a virtual host is a stupid choice for my application:(