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

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 16 Aug 2016 04:52:30 +0000
"Tian, Kevin" <kevin.tian@intel.com> wrote:

> > From: Neo Jia [mailto:cjia@nvidia.com]
> > Sent: Tuesday, August 16, 2016 12:17 PM
> > 
> > On Tue, Aug 16, 2016 at 03:50:44AM +0000, Tian, Kevin wrote:  
> > > > From: Neo Jia [mailto:cjia@nvidia.com]
> > > > 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:cjia@nvidia.com]
> > > > > > 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).  
> > 
> > Exactly, qemu is passed with the actual sysfs path.
> > 
> > So, QEMU doesn't touch the file /sys/class/mdev/mdev_start | mdev_stop at all.
> > 
> > QEMU will take the sysfs path as input:
> > 
> >  -device
> > vfio-pci,sysfsdev=/sys/bus/mdev/devices/c0b26072-dd1b-4340-84fe-bf338c510818-0,i
> > d=vgpu0  
> 
> no need of passing "id=vgpu0" here. If necessary you can put id as an attribute 
> under sysfs mdev node:
> 
> /sys/bus/mdev/devices/c0b26072-dd1b-4340-84fe-bf338c510818-0/id

QEMU needs an id parameter for devices, libvirt gives devices arbitrary
names, typically hostdev# for assigned devices.  This id is used to
reference the device for hmp/qmp commands.  This is not something the
mdev infrastructure should define.  Thanks,

Alex