[RFC] exposing 'nodedev assigned to domain' info to users

[Daniel Henrique Barboza]

Hi,

This is something I've been giving a thought after working in Gitlab issue
#72 and decided to run through the ML before hitting the code.

We don't have an easy way to retrieve the domain that is using an specific
hostdev.  Let's say that I want to know which domain is using the PCI card
pci_0000_01_00_2. 'nodedev-dumpxml' will return the hardware/driver capabilities
of the device, such as IOMMU group, driver and so on, but will not inform
which domain is using the hostdev, if any. 'nodedev-list' will simply list
all nodedev names known to Libvirt, without outputting any other information.

IIUC, the only existing way I can reliably tell whether a hostdev is being
used by domain, aside from having to register the information by myself
during domain definition of course, is via 'virsh dumpxml <domain>' each
existing running domain and matching the nodedev name with the source.address
element of the XML.

When we consider SR-IOV devices that can have 28+ VFs each (and have lots of
fun caveats, like Github #72 showed us), the capability of hot plug/unplug
hostdevs freely, and lots of running domains, it is clear that we're putting a
considerable pressure in the upper layers (OVirt, or a poor human admin) to
keep track of the nodedevs each running domain is using. An info that we
already have internally and can just expose it.

I have a few ideas to make this happen:

1 - upgrade 'nodedev-list' to add an extra 'assigned to' column

This is the more straightforward way of exposing the info. A simple 'nodedev-list'
call can retrieve which domain is using which nodedev. To preserve the existing
usage we can add an "--show-assigned-domains" option to control whether we
will display this info.


2 - add an '<assigned_to>' element in nodedev XML definition

I'm not a fan of exposing this in this particular XML because we would mix
host/hw related attributes with domain info. But it would be easier to pull
this off comparing to (1), so I'm mentioning it for the record.


I would start by exposing the info for HOSTDEV_SUBSYS_TYPE_PCI hostdevs
(--cap pci in nodedev-list).


Thanks,


DHB