Device assignment theoretical questions

Device assignment theoretical questions

[Wolfgang Nothdurft]

Hi,

what limits does kvm have with device assignment and vt-d enabled.

- Is it possible to assign pci devices behind the same pci bridge to
different guests?
- Is there a limit how many devices can be assigned to one guest?
- What kind of cards work with the dma=none option?

Regards
Wolfgang

RE: Device assignment theoretical questions

[Han, Weidong]

Wolfgang Nothdurft wrote:
> Hi,
> 
> what limits does kvm have with device assignment and vt-d enabled.
> 
> - Is it possible to assign pci devices behind the same pci bridge to
> different guests?

For devices behind conventional PCI bridges, the source-id in the DMA requests is the requester-id of the bridge device. For devices behind PCI Express-to-PCI/PCI-X bridges, the source-id possibly is the source-id of the original PCI-X transaction or the source-id provided by the bridge. So devices behind these bridges can only be collectively assigned to a single guest.

> - Is there a limit how many devices can be assigned to one guest?

VT-d doesn't have the limitation. kvm guest supports 32 devices.

> - What kind of cards work with the dma=none option?

dma=none implements DMA remapping using software method. I think it needn't depend on device. You can refer to Amit's pv-dma tree for more information.

Regards,
Weidong