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Subject: Re: [virtio-dev] Re: [RFC PATCH 2/3] netdev: kernel-only IFF_HIDDEN netdevice

On 4/18/2018 10:07 PM, Michael S. Tsirkin wrote:
On Wed, Apr 18, 2018 at 10:00:51PM -0700, Samudrala, Sridhar wrote:
On 4/18/2018 9:41 PM, Michael S. Tsirkin wrote:
On Wed, Apr 18, 2018 at 04:33:34PM -0700, Samudrala, Sridhar wrote:
On 4/17/2018 5:26 PM, Siwei Liu wrote:
I ran this with a few folks offline and gathered some good feedbacks
that I'd like to share thus revive the discussion.

First of all, as illustrated in the reply below, cloud service
providers require transparent live migration. Specifically, the main
target of our case is to support SR-IOV live migration via kernel
upgrade while keeping the userspace of old distros unmodified. If it's
because this use case is not appealing enough for the mainline to
adopt, I will shut up and not continue discussing, although
technically it's entirely possible (and there's precedent in other
implementation) to do so to benefit any cloud service providers.

If it's just the implementation of hiding netdev itself needs to be
improved, such as implementing it as attribute flag or adding linkdump
API, that's completely fine and we can look into that. However, the
specific issue needs to be undestood beforehand is to make transparent
SR-IOV to be able to take over the name (so inherit all the configs)
from the lower netdev, which needs some games with uevents and name
space reservation. So far I don't think it's been well discussed.

One thing in particular I'd like to point out is that the 3-netdev
model currently missed to address the core problem of live migration:
migration of hardware specific feature/state, for e.g. ethtool configs
and hardware offloading states. Only general network state (IP
address, gateway, for eg.) associated with the bypass interface can be
migrated. As a follow-up work, bypass driver can/should be enhanced to
save and apply those hardware specific configs before or after
migration as needed. The transparent 1-netdev model being proposed as
part of this patch series will be able to solve that problem naturally
by making all hardware specific configurations go through the central
bypass driver, such that hardware configurations can be replayed when
new VF or passthrough gets plugged back in. Although that
corresponding function hasn't been implemented today, I'd like to
refresh everyone's mind that is the core problem any live migration
proposal should have addressed.

If it would make things more clear to defer netdev hiding until all
functionalities regarding centralizing and replay are implemented,
we'd take advices like that and move on to implementing those features
as follow-up patches. Once all needed features get done, we'd resume
the work for hiding lower netdev at that point. Think it would be the
best to make everyone understand the big picture in advance before
going too far.
I think we should get the 3-netdev model integrated and add any additional
ndo_ops/ethool ops that we would like to support/migrate before looking into
hiding the lower netdevs.
Once they are exposed, I don't think we'll be able to hide them -
they will be a kernel ABI.

Do you think everyone needs to hide the SRIOV device?
Or that only some users need this?
Hyper-V is currently supporting live migration without hiding the SR-IOV device. So i don't
think it is a hard requirement.
OK, fine.

And also,  as we don't yet have a consensus on how to hide
the lower netdevs, we could make it as another feature bit to hide lower netdevs once
we have an acceptable solution.
Guest/host interface isn't more flexible than the userspace/kernel
interface.  The feature bit you propose would say what exactly?
Hypervisor has no idea what guest kernel shows guest userspace.
Note that the backup flag doesn't tell guest kernel what to do,
it just tells guest that there is or will be a faster main device
connected to the same backend, so the backup should only be used
when main device is not present.

The current bypass module supports 3-netdev and 2-netdev models via 2 sets of interfaces
bypass_master_create/destroy and bypass_master_register/unregister.  So theoretically
we can support the 2 models via 2 different feature bits. BACKUP and BACKUP_2_NETDEV.

Similarly if we can figure out a way to hide both the netdevs, we can add BACKUP_1_NETDEV
feature bit and update the bypass module to provide another set of interfaces that can
be used by virtio_net to support this model.

Now that we are leaning towards 'standby' as the name for the lower virtio-net, should we
change the feature bit name also to VIRTIO_NET_F_STANDBY?

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