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

On 1/6/21 7:09 AM, Daniel P. Berrangé wrote: > On Tue, Jan 05, 2021 at 05:18:13PM -0300, Daniel Henrique Barboza wrote: > > 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. > > That would mean nodedev-list has to fetch XML for every running guest > and parse and extract it. That's not a scalable solution. > > > 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. > > This is similar to what we do for the nwfilter-binding and net-port XML > where we have an <owner> element present. > > The complication here is that right now we don't ever touch the nodedev > driver when doing host device assignment, and so don't especially want > to introduce a dependancy. One possible alternative would be a new API that operates on hostdevs instead of nodedevs. "hostdev-list" would list the devices assigned to any domain, as opposed to "nodedev-list" that lists all nodedevs of the host. I'm not sure if this differentiation between hostdev and nodedev (i.e. hostdev is a nodedev that is assigned to a domain) would be clear enough to users though. We would need to document it clearer in the docs.

Yet another alternative is a new API under "Device Commands". We already have attach-device, detach-device and so on, might as well have a new "list-devices" that does the deed. This fits with the "The following commands manipulate devices associated to domains." claim that we make about this class of commands. Thanks, DHB > > > Regards, > Daniel >