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

On 1/26/2018 6:30 PM, Jakub Kicinski wrote:
On Fri, 26 Jan 2018 15:30:35 -0800, Samudrala, Sridhar wrote:
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
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
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.
Maybe you're not convinced but multiple arguments were made.
All the arguments seem to either saying that semantically this doesn't look like
a bond OR suggesting usecases that this patch is not trying to solve.
This approach should help cloud environments where the guest networking is fully controlled from the hypervisor via the PF driver or via port representor when switchdev mode is enabled. The guest admin is not expected or allowed to make any networking
changes from the VM.

With 2 netdev model, any VM image that has a working network
configuration will transparently get VF based acceleration without
any changes.
Nothing happens transparently.  Things may happen automatically.  The
VF netdev doesn't disappear with netvsc.  The PV netdev transforms into
something it did not use to be.  And configures and reports some
information from the PV (e.g. speed) but PV doesn't pass traffic any

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.
I don't understand this comment.  There is one virtio-net device and
one "virtio-bond" netdev.  And user space has to be aware of the special
automatic arrangement anyway, because it can't touch the VF.  It
doesn't make any difference whether it ignores the VF or PV and VF.
It simply can't touch the slaves, no matter how many there are.

If the userspace is not expected to touch the slaves, then why do we need to
take extra effort to expose a netdev that is just not really useful.

Also, from a user experience point of view, loading a virtio-net with
BACKUP feature enabled will now show 2 virtio-net netdevs.
One virtio-net and one virtio-bond, which represents what's happening.
This again assumes that we want to represent a bond setup. Can't we treat this as virtio-net providing an alternate low-latency datapath by taking over VF datapath?

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.

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