[libvirt] RFC: Creating mediated devices with libvirt

Alex Williamson alex.williamson at redhat.com
Fri Jun 16 17:02:55 UTC 2017

On Fri, 16 Jun 2017 11:32:04 -0400
Laine Stump <laine at redhat.com> wrote:

> On 06/15/2017 02:42 PM, Alex Williamson wrote:
> > On Thu, 15 Jun 2017 09:33:01 +0100
> > "Daniel P. Berrange" <berrange at redhat.com> wrote:
> >   
> >> On Thu, Jun 15, 2017 at 12:06:43AM +0200, Erik Skultety wrote:  
> >>> Hi all,
> >>>
> >>> so there's been an off-list discussion about finally implementing creation of
> >>> mediated devices with libvirt and it's more than desired to get as many opinions
> >>> on that as possible, so please do share your ideas. This did come up already as
> >>> part of some older threads ([1] for example), so this will be a respin of the
> >>> discussions. Long story short, we decided to put device creation off and focus
> >>> on the introduction of the framework as such first and build upon that later,
> >>> i.e. now.
> >>>
> >>> [1] https://www.redhat.com/archives/libvir-list/2017-February/msg00177.html
> >>>
> >>> ========================================
> >>> ========================================
> >>>
> >>> API-wise, device creation through the nodedev driver should be pretty
> >>> straightforward and without any issues, since virNodeDevCreateXML takes an XML
> >>> and does support flags. Looking at the current device XML:
> >>>
> >>> <device>
> >>>   <name>mdev_0cce8709_0640_46ef_bd14_962c7f73cc6f</name>
> >>>   <path>/sys/devices/pci0000:00/.../0cce8709-0640-46ef-bd14-962c7f73cc6f</path>
> >>>   <parent>pci_0000_03_00_0</parent>
> >>>   <driver>
> >>>     <name>vfio_mdev</name>
> >>>   </driver>
> >>>   <capability type='mdev'>
> >>>     <type id='nvidia-11'/>
> >>>     <iommuGroup number='13'/>
> >>>     <uuid>UUID<uuid> <!-- optional enhancement, see below -->
> >>>   </capability>
> >>> </device>
> >>>
> >>> We can ignore <path>,<driver>,<iommugroup> elements, since these are useless
> >>> during creation. We also cannot use <name> since we don't support arbitrary
> >>> names and we also can't rely on users providing a name in correct form which we
> >>> would need to further parse in order to get the UUID.
> >>> So since the only thing missing to successfully use create an mdev using XML is
> >>> the UUID (if user doesn't want it to be generated automatically), how about
> >>> having a <uuid> subelement under <capability> just like PCIs have <domain> and
> >>> friends, USBs have <bus> & <device>, interfaces have <address> to uniquely
> >>> identify the device even if the name itself is unique.
> >>> Removal of a device should work as well, although we might want to
> >>> consider creating a *Flags version of the API.
> >>>
> >>> =============================================================
> >>> =============================================================
> >>>
> >>> There were some doubts about auto-creation mentioned in [1], although they
> >>> weren't specified further. So hopefully, we'll get further in the discussion
> >>> this time.
> >>>
> >>> From my perspective there are two main reasons/benefits to that:
> >>>
> >>> 1) Convenience
> >>> For apps like virt-manager, user will want to add a host device transparently,
> >>> "hey libvirt, I want an mdev assigned to my VM, can you do that". Even for
> >>> higher management apps, like oVirt, even they might not care about the parent
> >>> device at all times and considering that they would need to enumerate the
> >>> parents, pick one, create the device XML and pass it to the nodedev driver, IMHO
> >>> it would actually	 be easier and faster to just do it directly through sysfs,
> >>> bypassing libvirt once again....    
> >>
> >> The convenience only works if the policy we've provided in libvirt actually
> >> matches the policy the application wants. I think it is quite likely that with
> >> cloud the mdevs will be created out of band from the domain startup process.
> >> It is possible the app will just have a fixed set of mdevs pre-created when
> >> the host starts up. Or that the mgmt app wants the domain startup process to
> >> be a two phase setup, where it first allocates the resources needed, and later
> >> then tries to start the guest. This is why I keep saying that putting this kind
> >> of "convenient" policy in libvirt is a bad idea - it is essentially just putting
> >> a bit of virt-manager code into libvirt - more advanced apps will need more
> >> flexibility in this area.
> >>  
> >>> 2) Future domain migration
> >>> Suppose now that the mdev backing physical devices support state dump and
> >>> reload. Chances are, that the corresponding mdev doesn't even exist or has a
> >>> different UUID on the destination, so libvirt would do its best to handle this
> >>> before the domain could be resumed.    
> >>
> >> This is not an unusual scenario - there are already many other parts of the
> >> device backend config that need to change prior to migration, especially for
> >> anything related to host devices, so apps already have support for doing
> >> this, which is more flexible & convenient becasue it doesn't tie creation of
> >> the mdevs to running of the migrate command.
> >>
> >> IOW, I'm still against adding any kind of automatic creation policy for
> >> mdevs in libvirt. Just provide the node device API support.  
> > 
> > I'm not super clear on the extent of what you're against here, is it
> > all forms of device creation or only a placement policy?  Are you
> > against any form of having the XML specify the non-instantiated mdev
> > that it wants?  We've clearly made an important step with libvirt
> > supporting pre-created mdevs, but as a user of that support I find it
> > incredibly tedious.  I typically do a dumpxml, copy out the UUID,
> > wonder what type of device it might have been last time, create it,
> > start the domain and cross my fingers. Pre-creating mdev devices is not
> > really practical, I might have use cases where I want multiple low-end
> > mdev devices and another where I have a single high-end device.  Those
> > cannot exist at the same time. Requiring extensive higher level
> > management tools is not really an option either, I'm not going to
> > install oVirt on my desktop/laptop just so I can launch a GVT-g VM once
> > in a while (no offense).  So I really hope that libvirt itself can
> > provide some degree of mdev creation.  
> Maybe there can be something in between the "all child devices must be
> pre-created" and "a child device will be automatically created on an
> automatically chosen parent device as needed". In particular, we could
> forego the "automatically chosen parent device" part of that. The guest
> configuration could simply contain the PCI address of the parent and the
> desired type of the child. If we did this there wouldn't be any policy
> decision to make - all the variables are determined - but it would make
> life easier for people running small hosts (i.e. no oVirt/Openstack, a
> single mdev parent device). Openstack and oVirt (and whoever) would of
> course be free to ignore this and pre-create pools of devices themselves
> in the name of more precise control and better predictability (just as,
> for example, OpenStack ignores libvirt's "pools of hostdev network
> devices" and instead manages the pool of devices itself and uses
> <interface type='hostdev'> directly).

This seems not that substantially different from managed='yes' on a
vfio hostdev to me.  It makes the device available to the VM before it
starts and returns it after.  In one case that's switching the binding
on an existing device, in another it's creating and removing.  Once
again, I can't tell from Dan's response if he's opposed to this entire
idea or just the aspects where libvirt needs to impose a policy
decision.  For me personally, the functionality difference is quite

> Of course this would mean that the "casual" user (one who just used
> virsh and virt-manager) would still need to find the mdev parent device
> and learn its PCI address and what child device types it supported. But
> those operations could be done once, the results recorded into the guest
> domain config, and thereafter forgotten about.

If libvirt had this minimal level of support, virt-manager could
"easily" add an option for adding mdev devices where it lists the
available parent devices and allows a list of mdev types to be expanded
and selected below each parent.  Clicky, clicky.  Thanks,


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