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 12:17 PM
> 
> On Tue, Aug 16, 2016 at 03:50:44AM +0000, Tian, Kevin wrote:
> > > 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).
> 
> 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

> 
> As you are saying in live migration, QEMU needs to access "start" and "stop". 
>  Could you
> please share more details, such as how QEMU access the "start" and "stop" 
> sysfs,
> when and what triggers that?
> 

A conceptual flow as below:

1. Quiescent mdev activity on the parent device (e.g. stop scheduling, wait for
in-flight DMA completed, etc.)

echo "0" > /sys/bus/mdev/devices/c0b26072-dd1b-4340-84fe-bf338c510818-0/start

2. Save mdev state:

cat /sys/bus/mdev/devices/c0b26072-dd1b-4340-84fe-bf338c510818-0/state > xxx

3. xxx will be part of the final VM image and copied to a new machine

4. Allocate/prepare mdev on the new machine for this VM

5. Restore mdev state:

cat xxx > /sys/bus/mdev/devices/c0b26072-dd1b-4340-84fe-bf338c510818-0/state
(might be a different path name)

6. start mdev on the new parent device:

echo "1" > /sys/bus/mdev/devices/c0b26072-dd1b-4340-84fe-bf338c510818-0/start

Thanks
Kevin