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

[Siwei Liu <loseweigh@gmail.com>]

On Wed, Apr 18, 2018 at 11:10 PM, Samudrala, Sridhar
<sridhar.samudrala@intel.com> wrote:
>
> 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.

I'm still trying to understand the value of so many models to support.
If we all agree eventually the transparent 1-netdev model can address
the more general case while 2-netdev or 3-netdev is unable to, what's
the point for supporting these many features?

-Siwei
>
> 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?
>
>>
>