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

lst_hoe02 at kwsoft.de lst_hoe02 at kwsoft.de
Thu Aug 21 18:43:13 CEST 2014

Zitat von Gilberto Nunes <gilberto.nunes32 at gmail.com>:

> http://en.wikipedia.org/wiki/X86_virtualization#I.2FO_MMU_virtualization_.28AMD-Vi_and_VT-d.29
> 2014-08-21 13:06 GMT-03:00 <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.

Hm, yes. But this still lead to the question which boot parameter  
should be used if any:

iommu=noaperture which looks like deaktivating GART IOMMU but also  
hardware IOMMU at all
amd_iommu which should activate AMD-Vi which includes IOMMU
no parameter which looks like using GART IOMMU but not AMD-Vi and only  
when IOMMU is enabled in BIOS

or do i simply not need it at all if no passthrough of devices is required?




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