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Re: [Qemu-devel] [PATCH v7 0/4] Add Mediated device support

From: Kirti Wankhede
Subject: Re: [Qemu-devel] [PATCH v7 0/4] Add Mediated device support
Date: Sat, 3 Sep 2016 00:03:55 +0530

On 9/2/2016 10:55 PM, Paolo Bonzini wrote:
> On 02/09/2016 19:15, Kirti Wankhede wrote:
>> On 9/2/2016 3:35 PM, Paolo Bonzini wrote:
>>>    <device>
>>>      <name>my-vgpu</name>
>>>      <parent>pci_0000_86_00_0</parent>
>>>      <capability type='mdev'>
>>>        <type id='11'/>
>>>        <uuid>0695d332-7831-493f-9e71-1c85c8911a08</uuid>
>>>      </capability>
>>>    </device>
>>> After creating the vGPU, if required by the host driver, all the other
>>> type ids would disappear from "virsh nodedev-dumpxml pci_0000_86_00_0" too.
>> Thanks Paolo for details.
>> 'nodedev-create' parse the xml file and accordingly write to 'create'
>> file in sysfs to create mdev device. Right?
>> At this moment, does libvirt know which VM this device would be
>> associated with?
> No, the VM will associate to the nodedev through the UUID.  The nodedev
> is created separately from the VM.
>>> When dumping the mdev with nodedev-dumpxml, it could show more complete
>>> info, again taken from sysfs:
>>>    <device>
>>>      <name>my-vgpu</name>
>>>      <parent>pci_0000_86_00_0</parent>
>>>      <capability type='mdev'>
>>>        <uuid>0695d332-7831-493f-9e71-1c85c8911a08</uuid>
>>>        <!-- only the chosen type -->
>>>        <type id='11'>
>>>          <!-- ... snip ... -->
>>>        </type>
>>>        <capability type='pci'>
>>>          <!-- no domain/bus/slot/function of course -->
>>>          <!-- could show whatever PCI IDs are seen by the guest: -->
>>>          <product id='...'>...</product>
>>>          <vendor id='0x10de'>NVIDIA</vendor>
>>>        </capability>
>>>      </capability>
>>>    </device>
>>> Notice how the parent has mdev inside pci; the vGPU, if it has to have
>>> pci at all, would have it inside mdev.  This represents the difference
>>> between the mdev provider and the mdev device.
>> Parent of mdev device might not always be a PCI device. I think we
>> shouldn't consider it as PCI capability.
> The <capability type='pci'> in the vGPU means that it _will_ be exposed
> as a PCI device by VFIO.
> The <capability type='pci'> in the physical GPU means that the GPU is a
> PCI device.

Ok. Got that.

>>> Random proposal for the domain XML too:
>>>   <hostdev mode='subsystem' type='pci'>
>>>     <source type='mdev'>
>>>       <!-- possible alternative to uuid: <name>my-vgpu</name> ?!? -->
>>>       <uuid>0695d332-7831-493f-9e71-1c85c8911a08</uuid>
>>>     </source>
>>>     <address type='pci' bus='0' slot='2' function='0'/>
>>>   </hostdev>
>> When user wants to assign two mdev devices to one VM, user have to add
>> such two entries or group the two devices in one entry?
> Two entries, one per UUID, each with its own PCI address in the guest.
>> On other mail thread with same subject we are thinking of creating group
>> of mdev devices to assign multiple mdev devices to one VM.
> What is the advantage in managing mdev groups?  (Sorry didn't follow the
> other thread).

When mdev device is created, resources from physical device is assigned
to this device. But resources are committed only when device goes
'online' ('start' in v6 patch)
In case of multiple vGPUs in a VM for Nvidia vGPU solution, resources
for all vGPU devices in a VM are committed at one place. So we need to
know the vGPUs assigned to a VM before QEMU starts.

Grouping would help here as Alex suggested in that mail. Pulling only
that part of discussion here:

<Alex> It seems then that the grouping needs to affect the iommu group
so that
> you know that there's only a single owner for all the mdev devices
> within the group.  IIRC, the bus drivers don't have any visibility
> to opening and releasing of the group itself to trigger the
> online/offline, but they can track opening of the device file
> descriptors within the group.  Within the VFIO API the user cannot
> access the device without the device file descriptor, so a "first
> device opened" and "last device closed" trigger would provide the
> trigger points you need.  Some sort of new sysfs interface would need
> to be invented to allow this sort of manipulation.
> Also we should probably keep sight of whether we feel this is
> sufficiently necessary for the complexity.  If we can get by with only
> doing this grouping at creation time then we could define the "create"
> interface in various ways.  For example:
> echo $UUID0 > create
> would create a single mdev named $UUID0 in it's own group.
> echo {$UUID0,$UUID1} > create
> could create mdev devices $UUID0 and $UUID1 grouped together.

I think this would create mdev device of same type on same parent
device. We need to consider the case of multiple mdev devices of
different types and with different parents to be grouped together.

<Alex> We could even do:
> echo $UUID1:$GROUPA > create
> where $GROUPA is the group ID of a previously created mdev device into
> which $UUID1 is to be created and added to the same group.

I was thinking about:

  echo $UUID0 > create

would create mdev device

  echo $UUID0 > /sys/class/mdev/create_group

would add created device to group.

For multiple devices case:
  echo $UUID0 > create
  echo $UUID1 > create

would create mdev devices which could be of different types and
different parents.
  echo $UUID0, $UUID1 > /sys/class/mdev/create_group

would add devices in a group.
Mdev core module would create a new group with unique number.  On mdev
device 'destroy' that mdev device would be removed from the group. When
there are no devices left in the group, group would be deleted. With
this "first device opened" and "last device closed" trigger can be used
to commit resources.
Then libvirt use mdev device path to pass as argument to QEMU, same as
it does for VFIO. Libvirt don't have to care about group number.


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