[libvirt] RFC: Check if the host device is assignable earlier

[Osier Yang]

On 2012年09月25日 00:32, Eric Blake wrote:
> On 09/23/2012 10:29 PM, Osier Yang wrote:
>> Hi,
>>
>> I think we already got enough bugs which are about "try to assign
>> a host device, then it fails for not assignable", clearly this
>> "try and then fails" approach is not nice.
>>
>> The reason for not assignable can be various. E.g.
>>
>>    * IOMMU or AMD-V is not enabled/supported
>>
>>    * platform doesn't support interrupt remapping
>>
>> There is no checking for above reasons now, an assignment will
>> get error till the qemu process started. It's nice if checking
>> it much earlier, and list whether the host supports device
>> assignment as a property of capabilities?
>
> Yes, failing earlier if it can be proven that the passthrough will not
> ever succeed seems worthwhile.
>
>> It will also be nice if one could known what devices are assignable
>> (list all assignable devices? this needs to enumerate and try
>> to unbind/reset/restore the devices, which is not desireable, as
>> it could breaks work on host, such as NIC). Or whether a device is
>> assignable (this shouldn't cause any problem when doing checking,
>> because it's the requested operation, one knowns what he does).
>>
>> Assuming a new API to determine whether a host device is assignable
>
> Why would we need a new API?  We can already get at each
> virNodeDevicePtr (see 'virsh nodedev-list')

New filtering flags for virNodeListAllDevices makes sense, but it
need to find a way to associate nodedev driver and the HV driver
together (it should be the time to do this now, there as similiar
request before as far as I could recall), to check whether the
device is being used by some guest.

and learn about each device
> (see 'virsh nodedev-dumpxml');
this seems like something that you would
> just add an addditional XML element

Yeah, we should have an new XML tag to expose the info anyway, but
for an upper layer app, it might be better to known if the device
is assignable directly, instead of parsing the device XML? (assuming
virsh attach-device will be one of the caller).

Regards,
Osier