Re: [pve-devel] [PATCH docs v4 4/5] added vIOMMU documentation

[Markus Frank <m.frank@proxmox.com>]

On 1/13/23 11:09, Wolfgang Bumiller wrote:
> On Fri, Nov 25, 2022 at 03:08:56PM +0100, Markus Frank wrote:
>> Signed-off-by: Markus Frank <m.frank@proxmox.com>
>> ---
>>   qm-pci-passthrough.adoc | 25 +++++++++++++++++++++++++
>>   1 file changed, 25 insertions(+)
>>
>> diff --git a/qm-pci-passthrough.adoc b/qm-pci-passthrough.adoc
>> index fa6ba35..7ed4d49 100644
>> --- a/qm-pci-passthrough.adoc
>> +++ b/qm-pci-passthrough.adoc
>> @@ -389,6 +389,31 @@ Example configuration with an `Intel GVT-g vGPU` (`Intel Skylake 6700k`):
>>   With this set, {pve} automatically creates such a device on VM start, and
>>   cleans it up again when the VM stops.
>>   
>> +[[qm_pci_viommu]]
>> +vIOMMU
>> +~~~~~~
>> +
>> +vIOMMU enables the option to passthrough pci devices to Level-2 VMs
>> +in Level-1 VMs via Nested Virtualisation.
>> +
>> +Host-Requirement: Set `intel_iommu=on` or `amd_iommu=on` depending on your
>> +CPU.
> 
> And by "CPU" you mean kernel command line? ;-)

Host-Requirement: Add `intel_iommu=on` or `amd_iommu=on`
depending on your CPU to your kernel command line.

like this?
> 
>> +
>> +VM-Requirement: For both Intel and AMD CPUs you will have to set
>> +`intel_iommu=on` as a Linux boot parameter in the vIOMMU-enabled-VM, because
>> +Qemu implements the Intel variant.
> 
> ^ As mentioned, there does appear to be an amd_iommu device in the qemu
> code, so would the amd variant work?
> 
> In my reply to the code patch I mentioned checking the host arch. But if
> you say we can use intel_iommu on AMD as well, I'd say, if both work,
> give the user a choice, otherwise we can of course just stick to the one
> that works ;-)

intel_iommu works better on my AMD CPU than amd_iommu ;)
Moreover it adds an extra AMDVI-PCI device that is using the first pci address.
`kvm: -device VGA,id=vga,bus=pcie.0,addr=0x1: PCI: slot 1 function 0 not available for VGA, in use by AMDVI-PCI,id=(null)`

I cannot find any good documentation for amd_iommu but it also seems like
it has less features.

$ qemu-system-x86_64 -device 'amd-iommu,help'
amd-iommu options:
   device-iotlb=<bool>    -  (default: false)
   intremap=<OnOffAuto>   - on/off/auto (default: "auto")
   pt=<bool>              -  (default: true)
$ qemu-system-x86_64 -device 'intel-iommu,help'
intel-iommu options:
   aw-bits=<uint8>        -  (default: 39)
   caching-mode=<bool>    -  (default: false)
   device-iotlb=<bool>    -  (default: false)
   dma-drain=<bool>       -  (default: true)
   dma-translation=<bool> -  (default: true)
   eim=<OnOffAuto>        - on/off/auto (default: "auto")
   intremap=<OnOffAuto>   - on/off/auto (default: "auto")
   pt=<bool>              -  (default: true)
   snoop-control=<bool>   -  (default: false)
   version=<uint32>       -  (default: 0)
   x-buggy-eim=<bool>     -  (default: false)
   x-pasid-mode=<bool>    -  (default: false)
   x-scalable-mode=<bool> -  (default: false)