Re: [Qemu-devel] [PATCH v6 1/4] vfio: Mediated device Core driver

[Tian, Kevin]

> From: Neo Jia [mailto:address@hidden
> Sent: Tuesday, August 16, 2016 7:24 AM
> 
> > > >
> > > > > >
> > > > > > For NVIDIA vGPU solution we need to know all devices assigned to a 
> > > > > > VM in
> > > > > > one shot to commit resources of all vGPUs assigned to a VM along 
> > > > > > with
> > > > > > some common resources.
> > > > >
> > > > > Kirti, can you elaborate the background about above one-shot commit
> > > > > requirement? It's hard to understand such a requirement.
> > > >
> > > > Agree, I know NVIDIA isn't planning to support hotplug initially, but
> > > > this seems like we're precluding hotplug from the design.  I don't
> > > > understand what's driving this one-shot requirement.
> > >
> > > Hi Alex,
> > >
> > > The requirement here is based on how our internal vGPU device model 
> > > designed and
> > > with this we are able to pre-allocate resources required for multiple 
> > > virtual
> > > devices within same domain.
> > >
> > > And I don't think this syntax will stop us from supporting hotplug at all.
> > >
> > > For example, you can always create a virtual mdev and then do
> > >
> > > echo "mdev_UUID" > /sys/class/mdev/mdev_start
> > >
> > > then use QEMU monitor to add the device for hotplug.
> >
> > Hi Neo,
> >
> > I'm still not understanding the advantage you get from the "one-shot"
> > approach then if we can always add more mdevs by starting them later.
> > Are the hotplug mdevs somehow less capable than the initial set of
> > mdevs added in a single shot?  If the initial set is allocated
> > from the "same domain", does that give them some sort of hardware
> > locality/resource benefit?  Thanks,
> 
> Hi Alex,
> 
> At least we will not able to guarantee some special hardware resource for the
> hotplug devices.
> 
> So from our point of view, we also have dedicated internal SW entity to 
> manage all
> virtual devices for each "domain/virtual machine", and such SW entity will be 
> created
> at virtual device start time.
> 
> This is why we need to do this in one-shot to support multiple virtual device
> per VM case.
> 

Is pre-allocation of special hardware resource done one-time for all mdev 
instances?
Can it be done one-by-one as long as mdev is started early before VM is 
launched?

If such one-shot requirement is really required, it would be cleaner to me to
introduce a mdev group concept, so mdev instances with one-short start 
requirements can be put under a mdev group. Then you can do one-shot start
by:

echo "0/1" > /sys/class/mdev/group/0/start

Thanks
Kevin