[PVE-User] IOMMU and AMD-Vi useful/recommended for Proxmox ?

[lst_hoe02 at kwsoft.de]

Hello,

when inspecting the "dmesg" output on one of the PVE servers i got the  
follwoing:

No AGP bridge found
Node 0: aperture @ 0 size 32 MB
Your BIOS doesn't leave a aperture memory hole
Please enable the IOMMU option in the BIOS setup
This costs you 64 MB of RAM
Mapping aperture over 65536 KB of RAM @ 20000000
PM: Registered nosave memory: 0000000020000000 - 0000000024000000
AMD-Vi disabled by default: pass amd_iommu=on to enable

My first action was to go to the BIOS (the machine is still test) and  
activated the IOMMU. The message stays the same. Next try was to set  
iommu=noaperture as boot parameter because it was suggested for  
machine having IOMMU but no AGP anymore. This lead to

PCI-DMA: Using software bounce buffering for IO (SWIOTLB)
Placing 64MB software IO TLB between ffff880020000000 - ffff880024000000
software IO TLB at phys 0x20000000 - 0x24000000

Which looks like the IOMMU emulated in software...

This lead to the question what the real benefits of AMD-Vi / IOMMU are  
for Proxmox/KVM if they are disabled by default and can not be enabled  
straight forward as it looks like?

The Board in question is a ASUS KCMA-D8, BIOS 3102 with two AMD  
Opteron 4226 CPUs.

Thanks for any hints

Andreas