[openstack-dev] [nova][pci-passthrough] definitely specify VFIO driver as the host PCI driver for passthrough

Chen Fan fan.chen at easystack.cn
Fri Jun 24 08:52:31 UTC 2016

On 2016年06月24日 16:20, Daniel P. Berrange wrote:
> On Fri, Jun 24, 2016 at 12:27:57PM +0800, Chen Fan wrote:
>> hi all,
>>       in openstack, we can use the pci passthrough feature now, refer to
>>       https://wiki.openstack.org/wiki/Pci_passthrough
>>       but we can't definitely specify the host pci driver is LEGACY_KVM or
>> newer VFIO,
>>       new VFIO driver is more safer, and higher-performance user-space driver
>>       than legacy kvm driver (pci-stub), the benefit relative to kvm
>> assignment driver
>>       could refer to http://lwn.net/Articles/474088/.
>>       In additional, VFIO driver provides the GPU passthrough as primary card
>> support.
>>       I think it is more useful for further GPU passthrough support in
>> openstack.
>>       Openstack relies on the libvirt nodedev device configuration to do pci
>> passthrough,
>>       with managed mode, the configured device is automatically detached and
>> re-attached
>>       with KVM or VFIO driver that depended on the host driver modules
>> configuration,
>>       so now we can't specify the driver in openstack to VFIO mode, I think
>> we should need
>>       to add this feature support in openstack to get pci passhthrough more
>> scalability.
>>       a simply idea is to add a option in nova.conf HOST_PCI_MODEL = VFIO
>> /KVM to specify
>>       the pci passthrough device driver is using VFIO driver.
>>       any comments are welcome. :)
> I don't see any reason to add a configuration option. If the host is
> capable of doing VFIO, libvirt will always aim to use VFIO in preference
> to the legacy system.
Hi Daniel,

     sorry, I directly reference the implementation of nodedev in 
libvirt, in function
virHostdevPreparePCIDevices :
  257         if (pcisrc->backend == VIR_DOMAIN_HOSTDEV_PCI_BACKEND_VFIO)
  258             virPCIDeviceSetStubDriver(pci, VIR_PCI_STUB_DRIVER_VFIO);
  259         else if (pcisrc->backend == 
  260             virPCIDeviceSetStubDriver(pci, VIR_PCI_STUB_DRIVER_XEN);
  261         else
  262             virPCIDeviceSetStubDriver(pci, VIR_PCI_STUB_DRIVER_KVM);

IIUC, the stub driver should be "legacy KVM", do we need to change the 
stub drvier


> Regards,
> Daniel

Chen Fan

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