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

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

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. 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.


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