[libvirt] [RFC] handling hostdev save/load net config for non SR-IOV devices

[Laine Stump]

On 7/18/19 10:29 AM, Daniel Henrique Barboza wrote:
> Hi,
>
> I have a PoC that enables partial coldplug assignment of multifunction
> PCI devices with managed mode. At this moment, Libvirt can't handle
> this scenario - the code will detach only the hostdevs from the XML,
> when in fact the whole IOMMU needs to be detached. This can be
> verified by the fact that Libvirt handles the unmanaged scenario
> well, as long as the user detaches the whole IOMMU beforehand.
>
> I have played with 2 approaches. The one I am planning to contribute
> back is a change inside virHostdevGetPCIHostDeviceList(), that
> adds the extra PCI devices for detach/re-attach in case a PCI
> Multifunction device in managed mode is presented in the XML.


If you're thinking of doing that automatically, then I should warn you 
that we had discussed that a long time ago, and decided that it was a 
bad idea to do it because it was likely someone would, e.g. try to 
assign an audio device to their guest that happened to be one function 
on a multifunction device that also contained a disk controller (or some 
other device) that the host needed for proper operation.


It may be that in *your* particular case, you understand that the 
functions you don't want to assign to the guest are not otherwise used, 
and it's not dangerous to suddenly detach them from their host driver. 
But you can't assume that will always be the case.


If you *really* can't accept just assigning all the devices in that 
IOMMU group to the guest (thus making them all explicitly listed in the 
config, and obvious to the administrator that they won't be available on 
the host) and simply not using them, then you either need to separately 
detach those particular functions from the host, or come up with a way 
of having the domain config explicitly list them as "detached from the 
host but not actually attached to the guest".



> Now, there's a catch. Inside both virHostdevPreparePCIDevices()
> and virHostdevReAttachPCIDevices() there are code to save/restore
> the network configuration for SR-IOV devices. These functions iterates
> in the hostdevs list, instead of the pcidevs list I'm changing. The final
> result, given that the current conditions used for SR-IOV matches the
> conditions for multifunction PCI devices as well, is that not all virtual
> functions will get their network configuration saved/restored.


If you're not going to use a device (which is implied by the fact that 
it's not in the hostdevs list) then nothing about its network config 
will change, so there is no reason to save/restore it.


>
> For example, a guest that uses 3 of 4 functions of a PCI MultiFunction
> card, let's say functions 0,1 and 3. The code will handle the detach
> of all the IOMMU, including the function 2 that isn't declared in the
> XML.


Again, the above sentence implies that you're wanting to make this 
completely automatic, which we previously decided was something we 
didn't want to do.


> However, since function 2 isn't a hostdev, its network config
> will not be restored after the VM shutdown.


You're talking about something that will never occur - on every SRIOV 
network card I've ever seen each VF is in its own IOMMU group, and can 
be assigned to a guest independent of what's done with any other VF. 
I've never seen a case (except maybe once with a newly released 
motherboard that had broken IOMMU firmware(?)) where a VF was in the 
same IOMMU group as any other device.


>
> Now comes the question: how much effort should be spent into making
> the network config of all the functions be restored? Is this a blocker
> for the whole code to be accepted or, given it is proper documented
> somewhere, it can be done later on?

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listman.redhat.com/archives/libvir-list/attachments/20190718/dcbb1db1/attachment-0001.htm>