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I have had troubles installing Ubuntu on a new computer with an MSI B450 board and a RYZEN 5 3600.

First tried with Ubuntu 20.04 then downgraded my stick to 18.04 but still unsuccessful at first.

I finally managed to do it with 2 different SO threads :

Unable to install Ubuntu 18.04 on AMD Ryzen 2500U with Radeon Vega 8 Gfx

https://unix.stackexchange.com/questions/519758/amd-vi-completion-wait-loop-after-a-failed-install-of-arch-linux-on-a-blank-ssd

I had to add to my grub : nosplash nomodeset noacpi=off and from the latter thread iommu=off

Then I have been able to install UBUNTU 18.04 properly on my SSD.

Yet I have now to manually add iommu=off when booting.. or Ubuntu will not launch and I will get a purple blank screen.

I guess I should tweak my GRUB for good. Yet I am not even sure what IOMMU is for ? and if iommu=off is the actual solution.. I have read Iommu was about the graphics (my graphic card is indeed an AMD Radeon 550 2GB).

I really don't care the graphics as I bought this graphic card just to be able to connect a screen to the computer (I don't play games). Yet I'd like to know if adding iommu=off will somehow snowball to other parts of the computer / decrease performance or create instability whatsoever .

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    Did you check with the vendor for any firmware updates? I'd expect the 20.04 kernel would be more current than the 18.04 kernel (for awhile at least until a hwe update). Check inxi output (-C shows cores/threads) -- some acpi switches force only using one core.
    – ubfan1
    Jul 22, 2020 at 22:12
  • Hi @ubfan1 It seams the hyperthreading is working good, I can see the 12 cores. Also I have learnt more about Iommu: it helps passing a graphic card performance to a virtual machine. As I don't do this at all I will leave the Grub with iommu=off. I couldn't find anywhere in the BIOS any mention to Iommu. And I have updated back to Ubuntu 20.04 also. It works like a charm and boots much fatser than 18.04. So all good so far.
    – Maxence
    Jul 23, 2020 at 11:18

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