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 11:46 AM
> 
> On Tue, Aug 16, 2016 at 12:30:25AM +0000, Tian, Kevin wrote:
> > > From: Neo Jia [mailto:address@hidden
> > > Sent: Tuesday, August 16, 2016 3:59 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.
> > > >
> > > > As I relied in another mail, I really hope start/stop become a per-mdev
> > > > attribute instead of global one, e.g.:
> > > >
> > > > echo "0/1" > /sys/class/mdev/12345678-1234-1234-1234-123456789abc/start
> > > >
> > > > In many scenario the user space client may only want to talk to mdev
> > > > instance directly, w/o need to contact its parent device. Still take
> > > > live migration for example, I don't think Qemu wants to know parent
> > > > device of assigned mdev instances.
> > >
> > > Hi Kevin,
> > >
> > > Having a global /sys/class/mdev/mdev_start doesn't require anybody to know
> > > parent device. you can just do
> > >
> > > echo "mdev_UUID" > /sys/class/mdev/mdev_start
> > >
> > > or
> > >
> > > echo "mdev_UUID" > /sys/class/mdev/mdev_stop
> > >
> > > without knowing the parent device.
> > >
> >
> > You can look at some existing sysfs example, e.g.:
> >
> > echo "0/1" > /sys/bus/cpu/devices/cpu1/online
> >
> > You may also argue why not using a global style:
> >
> > echo "cpu1" > /sys/bus/cpu/devices/cpu_online
> > echo "cpu1" > /sys/bus/cpu/devices/cpu_offline
> >
> > There are many similar examples...
> 
> Hi Kevin,
> 
> My response above is to your question about using the global sysfs entry as 
> you
> don't want to have the global path because
> 
> "I don't think Qemu wants to know parent device of assigned mdev instances.".
> 
> So I just want to confirm with you that (in case you miss):
> 
>     /sys/class/mdev/mdev_start | mdev_stop
> 
> doesn't require the knowledge of parent device.
> 

Qemu is just one example, where your explanation of parent device
makes sense but still it's not good for Qemu to populate /sys/class/mdev
directly. Qemu is passed with the actual sysfs path of assigned mdev
instance, so any mdev attributes touched by Qemu should be put under 
that node (e.g. start/stop for live migration usage as I explained earlier).

Thanks
Kevin