Date:   Wed, 18 Apr 2018 23:10:05 -0700
From:   "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Siwei Liu <loseweigh@...il.com>,
        David Miller <davem@...emloft.net>,
        David Ahern <dsahern@...il.com>, Jiri Pirko <jiri@...nulli.us>,
        si-wei liu <si-wei.liu@...cle.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        Jakub Kicinski <kubakici@...pl>,
        Jason Wang <jasowang@...hat.com>,
        Netdev <netdev@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        virtio-dev@...ts.oasis-open.org
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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