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

[Neo Jia <cjia@nvidia.com>]

On Tue, Aug 16, 2016 at 04:52:30AM +0000, Tian, Kevin 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

I think we have moved away from the device index based on Alex's comment, so the
device path will be:

 /sys/bus/mdev/devices/c0b26072-dd1b-4340-84fe-bf338c510818

> 
> > 
> > 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 for the sequence, so based on above live migration, the access of "start/stop"
are from other user space program not QEMU process.

(Just to be clear, I am not saying that I won't consider your suggestion of 
accommodating the "start/stop" file from global to mdev node, but I do want to point 
out that keeping them inside global shouldn't impact your live migration sequence 
above.)

Thanks,
Neo

> 
> Thanks
> Kevin