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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v7 0/4] Add Mediated device support
Date: Fri, 2 Sep 2016 19:25:18 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0

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.

>> 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).


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