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Re: [Qemu-devel] [PATCH v6 1/4] vfio: Mediated device Core driver
From: |
Tian, Kevin |
Subject: |
Re: [Qemu-devel] [PATCH v6 1/4] vfio: Mediated device Core driver |
Date: |
Tue, 16 Aug 2016 05:58:54 +0000 |
> From: Neo Jia [mailto:address@hidden
> Sent: Tuesday, August 16, 2016 1:44 PM
>
> On Tue, Aug 16, 2016 at 04:52:30AM +0000, Tian, Kevin wrote:
> > > 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
>
> 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
pass /sys/bus/mdev/devices/c0b26072-dd1b-4340-84fe-bf338c510818
as parameter, and then Qemu can access 'id' under that path. You
don't need to pass a separate 'id' field. That's my point.
>
> >
> > >
> > > 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.)
>
come on... I just use above bash command to show the step. Qemu itself
definitely
needs to open the sysfs file descriptor and then read/write the fd... If we use
VFIO
API as example, it might be more obvious. :-)
Thanks
Kevin
- Re: [Qemu-devel] [PATCH v6 1/4] vfio: Mediated device Core driver, (continued)
- Re: [Qemu-devel] [PATCH v6 1/4] vfio: Mediated device Core driver, Tian, Kevin, 2016/08/15
- Re: [Qemu-devel] [PATCH v6 1/4] vfio: Mediated device Core driver, Neo Jia, 2016/08/15
- Re: [Qemu-devel] [PATCH v6 1/4] vfio: Mediated device Core driver, Tian, Kevin, 2016/08/15
- Re: [Qemu-devel] [PATCH v6 1/4] vfio: Mediated device Core driver, Neo Jia, 2016/08/16
- Re: [Qemu-devel] [PATCH v6 1/4] vfio: Mediated device Core driver, Tian, Kevin, 2016/08/16
- Re: [Qemu-devel] [PATCH v6 1/4] vfio: Mediated device Core driver, Neo Jia, 2016/08/16
- Re: [Qemu-devel] [PATCH v6 1/4] vfio: Mediated device Core driver,
Tian, Kevin <=
- Re: [Qemu-devel] [PATCH v6 1/4] vfio: Mediated device Core driver, Neo Jia, 2016/08/16
- Re: [Qemu-devel] [PATCH v6 1/4] vfio: Mediated device Core driver, Alex Williamson, 2016/08/16
- Re: [Qemu-devel] [PATCH v6 1/4] vfio: Mediated device Core driver, Alex Williamson, 2016/08/16
[Qemu-devel] [PATCH v6 3/4] vfio iommu: Add support for mediated devices, Kirti Wankhede, 2016/08/03
[Qemu-devel] [PATCH v6 4/4] docs: Add Documentation for Mediated devices, Kirti Wankhede, 2016/08/03