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

Daniel Henrique Barboza <danielhb413@xxxxxxxxx> · Thu, 18 Jul 2019 11:29:17 -0300



    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.

      
      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.

      
      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. However, since function 2 isn't a hostdev, its network config

      will not be restored after the VM shutdown.

      
      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?

      
      Thanks,

      
      DHB 

      
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