Re: [Qemu-devel] [libvirt] [PATCH v7 0/4] Add Mediated device support

[Laine Stump]

On 09/02/2016 05:44 PM, Paolo Bonzini wrote:
> On 02/09/2016 22:19, John Ferlan wrote:
> > We don't have such a pool for GPU's (yet) - although I suppose they
> > could just become a class of storage pools.
> >
> > The issue being nodedev device objects are not saved between reboots.
> > They are generated on the fly. Hence the "create-nodedev' API - notice
> > there's no "define-nodedev' API, although I suppose one could be
> > created. It's just more work to get this all to work properly.
>
> It can all be made transient to begin with.  The VM can be defined but
> won't start unless the mdev(s) exist with the right UUIDs.
>
> > > After creating the vGPU, if required by the host driver, all the other
> > > type ids would disappear from "virsh nodedev-dumpxml pci_0000_86_00_0" too.
> >
> > Not wanting to make assumptions, but this reads as if I create one type
> > 11 vGPU, then I can create no others on the host.  Maybe I'm reading it
> > wrong - it's been a long week.
>
> Correct, at least for NVIDIA.
>
> > PCI devices have the "managed='yes|no'" attribute as well. That's what
> > determines whether the device is to be detached from the host or not.
> > That's been something very painful to manage for vfio and well libvirt!
>
> mdevs do not exist on the host (they do not have a driver on the host
> because they are not PCI devices) so they do need any management.  At
> least I hope that's good news. :)

What's your definition of "management"? They don't need the same type of 
management as a traditional hostdev, but they certainly don't just 
appear by magic! :-)

For standard PCI devices, the managed attribute says whether or not the 
device needs to be detached from the host driver and attached to 
vfio-pci. For other kinds of hostdev devices, we could decide that it 
meant something different. In this case, perhaps managed='yes' could 
mean that the vGPU will be created as needed, and destroyed when the 
guest is finished with it, and managed='no' could mean that we expect a 
vGPU to already exist, and just need starting.

Or not. Maybe that's a pointless distinction in this case. Just pointing 
out the option...