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

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On 9/3/2016 5:27 AM, Laine Stump wrote:
> On 09/02/2016 05:44 PM, Paolo Bonzini wrote:
>>
>>
>> On 02/09/2016 22:19, John Ferlan wrote:
>>> We don't have such a pool for GPU's (yet) - although I suppose they
>>> could just become a class of storage pools.
>>>
>>> The issue being nodedev device objects are not saved between reboots.
>>> They are generated on the fly. Hence the "create-nodedev' API - notice
>>> there's no "define-nodedev' API, although I suppose one could be
>>> created. It's just more work to get this all to work properly.
>>
>> It can all be made transient to begin with.  The VM can be defined but
>> won't start unless the mdev(s) exist with the right UUIDs.
>>
>>>> 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.
>>>
>>> Not wanting to make assumptions, but this reads as if I create one type
>>> 11 vGPU, then I can create no others on the host.  Maybe I'm reading it
>>> wrong - it's been a long week.
>>
>> Correct, at least for NVIDIA.
>>
>>> PCI devices have the "managed='yes|no'" attribute as well. That's what
>>> determines whether the device is to be detached from the host or not.
>>> That's been something very painful to manage for vfio and well libvirt!
>>
>> mdevs do not exist on the host (they do not have a driver on the host
>> because they are not PCI devices) so they do need any management.  At
>> least I hope that's good news. :)
> 
> What's your definition of "management"? They don't need the same type of
> management as a traditional hostdev, but they certainly don't just
> appear by magic! :-)
> 
> For standard PCI devices, the managed attribute says whether or not the
> device needs to be detached from the host driver and attached to
> vfio-pci. For other kinds of hostdev devices, we could decide that it
> meant something different. In this case, perhaps managed='yes' could
> mean that the vGPU will be created as needed, and destroyed when the
> guest is finished with it, and managed='no' could mean that we expect a
> vGPU to already exist, and just need starting.
> 
> Or not. Maybe that's a pointless distinction in this case. Just pointing
> out the option...
> 

Mediated devices are like virtual device, there could be no direct
physical device associated with it. All mdev devices are owned by
vfio_mdev module, which is similar to vfio_pci module. I don't think we
need  to interpret 'managed' attribute for mdev devices same as standard
PCI devices.
If mdev device is created, you would find the device directory in
/sys/bus/mdev/devices/ directory.

Kirti.

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