[openstack-dev] [nova] [neutron] PCI pass-through network support

yongli he yongli.he at intel.com
Thu Jan 16 08:07:25 UTC 2014

On 2014?01?16? 08:28, Ian Wells wrote:
> To clarify a couple of Robert's points, since we had a conversation 
> earlier:
> On 15 January 2014 23:47, Robert Li (baoli) <baoli at cisco.com 
> <mailto:baoli at cisco.com>> wrote:
>     ---  do we agree that BDF address (or device id, whatever you call
>     it), and node id shouldn't be used as attributes in defining a PCI
>     flavor?
> Note that the current spec doesn't actually exclude it as an option.  
> It's just an unwise thing to do.  In theory, you could elect to define 
> your flavors using the BDF attribute but determining 'the card in this 
> slot is equivalent to all the other cards in the same slot in other 
> machines' is probably not the best idea...  We could lock it out as an 
> option or we could just assume that administrators wouldn't be daft 
> enough to try.
>           * the compute node needs to know the PCI flavor. [...]
>                       - to support live migration, we need to use it
>     to create network xml
> I didn't understand this at first and it took me a while to get what 
> Robert meant here.
> This is based on Robert's current code for macvtap based live 
> migration.  The issue is that if you wish to migrate a VM and it's 
> tied to a physical interface, you can't guarantee that the same 
> physical interface is going to be used on the target machine, but at 
> the same time you can't change the libvirt.xml as it comes over with 
> the migrating machine.  The answer is to define a network and refer 
> out to it from libvirt.xml.  In Robert's current code he's using the 
> group name of the PCI devices to create a network containing the list 
> of equivalent devices (those in the group) that can be macvtapped.  
> Thus when the host migrates it will find another, equivalent, 
> interface. This falls over in the use case under
but, with flavor we defined, the group could be a tag for this purpose, 
and all Robert's design still work, so it ok, right?
> consideration where a device can be mapped using more than one flavor, 
> so we have to discard the use case or rethink the implementation.
> There's a more complex solution - I think - where we create a 
> temporary network for each macvtap interface a machine's going to use, 
> with a name based on the instance UUID and port number, and containing 
> the device to map. Before starting the migration we would create a 
> replacement network containing only the new device on the target host; 
> migration would find the network from the name in the libvirt.xml, and 
> the content of that network would behave identically.  We'd be 
> creating libvirt networks on the fly and a lot more of them, and we'd 
> need decent cleanup code too ('when freeing a PCI device, delete any 
> network it's a member of'), so it all becomes a lot more hairy.
> -- 
> Ian.
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