Re: [virtio-dev] Re: [RFC PATCH net-next v2 2/2] virtio_net: Extend virtio to use VF datapath when available

["Samudrala, Sridhar" <sridhar.samudrala@intel.com>]

On 1/26/2018 2:47 PM, Jakub Kicinski wrote:
> On Sat, 27 Jan 2018 00:14:20 +0200, Michael S. Tsirkin wrote:
> > On Fri, Jan 26, 2018 at 01:46:42PM -0800, Siwei Liu wrote:
> > > > and the VM is not expected to do any tuning/optimizations on the VF driver
> > > > directly,
> > > > i think the current patch that follows the netvsc model of 2 netdevs(virtio
> > > > and vf) should
> > > > work fine.
> > > OK. For your use case that's fine. But that's too specific scenario
> > > with lots of restrictions IMHO, perhaps very few users will benefit
> > > from it, I'm not sure. If you're unwilling to move towards it, we'd
> > > take this one and come back with a generic solution that is able to
> > > address general use cases for VF/PT live migration .
> > I think that's a fine approach. Scratch your own itch!  I imagine a very
> > generic virtio-switchdev providing host routing info to guests could
> > address lots of usecases. A driver could bind to that one and enslave
> > arbitrary other devices.  Sounds reasonable.
> >
> > But given the fundamental idea of a failover was floated at least as
> > early as 2013, and made 0 progress since precisely because it kept
> > trying to address more and more features, and given netvsc is already
> > using the basic solution with some success, I'm not inclined to block
> > this specific effort waiting for the generic one.
> I think there is an agreement that the extra netdev will be useful for
> more advanced use cases, and is generally preferable.  What is the
> argument for not doing that from the start?  If it was made I must have
> missed it.  Is it just unwillingness to write the extra 300 lines of
> code?  Sounds like a pretty weak argument when adding kernel ABI is at
> stake...

I am still not clear on the need for the extra netdev created by 
virtio_net. The only advantage
i can see is that the stats can be broken between VF and virtio 
datapaths compared
to the aggregrated stats on virtio netdev as seen with the 2 netdev 
approach.

With 2 netdev model, any VM image that has a working network 
configuration will transparently get
VF based acceleration without any changes. 3 netdev model breaks this 
configuration starting with the
creation and naming of the 2 devices to udev needing to be aware of 
master and slave virtio-net devices.
Also, from a user experience point of view, loading a virtio-net with 
BACKUP feature
enabled will  now show 2 virtio-net netdevs.

For live migration with advanced usecases that Siwei is suggesting, i 
think we need a new driver
with a new device type that can track the VF specific feature settings 
even when the VF driver is unloaded.

Thanks
Sridhar