Re: Re: [PATCH v4 2/2] virtio_net: Extend virtio to use VF datapath when available

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

On 3/7/2018 12:11 PM, Michael S. Tsirkin wrote:
> On Wed, Mar 07, 2018 at 10:06:30AM -0800, Stephen Hemminger wrote:
>> On Wed, 7 Mar 2018 09:50:50 -0800
>> Alexander Duyck <alexander.duyck@gmail.com> wrote:
>>
>>> On Tue, Mar 6, 2018 at 6:38 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
>>>> On Tue, Mar 06, 2018 at 03:27:46PM -0800, Alexander Duyck wrote:
>>>>>> I definitelly vote for a separate common shared code for both netvsc and
>>>>>> virtio_net - even if you use 2 and 3 netdev model, you could share the
>>>>>> common code. Strict checks and limitation should be in place.
>>>>> Noted. But as I also mentioned there isn't that much "common" code
>>>>> between the two models. I think if anything we could probably look at
>>>>> peeling out a few bits such as "get_<iface>_bymac" which really would
>>>>> become dev_get_by_mac_and_ops in order to find the device for the
>>>>> notifiers. I probably wouldn't even put that in our driver and would
>>>>> instead put it in the core code since it almost makes more sense
>>>>> there. Beyond that sharing becomes much more challenging due to the
>>>>> differences in the Rx and Tx paths that build out of the difference
>>>>> between the 2 driver and 3 driver models.
>>>> At this point it might be worth it to articulate the advantages
>>>> of the 3 netdev model.
>>> The main advantages are performance on the lower devices. Specifically
>>> we can run LLTX and IFF_NOQUEUE on the upper device, meaning the VF
>>> doesn't take a performance hit when we start transmitting through it.
>>> In addition the paravirtual device is able to fully expose it's
>>> features, so for example virtio_net can maintain in-driver XDP, and we
>>> can provide generic XDP on the upper device. We can also have an
>>> asymmetric configuration where the number of queues or feature sets
>>> can be different between the ports.
>>>
>>> Basically what it comes down to is in the 3 netdev model we never have
>>> to deal with the issues of going from being a standard netdev to being
>>> a master netdev. The role of each netdev is clearly defined.
>>>
>>> As far as doing a generic solution it is the preferred way to go as
>>> well since we don't have to rewrite functionality of the lower device.
>>> Currently the only changes really needed are to add a phys_port_name
>>> operation so that you can distinguish between the bypass interface and
>>> the original paravirtual one. As such you have no confusion about what
>>> you are running.
>>>
>>>> If they are compelling, why wouldn't netvsc users want them?
>>> I believe part of the logic for Stephen's choice is that netvsc
>>> doesn't have a "BACKUP" bit like what we have virtio_net. As a result
>>> they have no way of knowing if a VF will ever show up or not. That
>>> makes the 3 netdev solution less appealing as they always end up in
>>> the bonding mode. In addition they have intentionally limited
>>> themselves to avoid features such as XDP that would cause issues with
>>> the 2 netdev model.
>>>
>>>> Alex, I think you were one of the strongest proponents of this model,
>>>> you should be well placed to provide a summary.
>>> Hopefully the information provided helps. In my mind the issue is that
>>> netvsc was not designed correctly, and as such it is using the 2
>>> netdev model as a bit of a crutch to handle the case where they wanted
>>> to add a VF bypass. At this point it has become a part of their
>>> architecture so it would be confusing for their user base to deal with
>>> having 2 netdevs spawn every time their driver sees a new device. In
>>> the case of virtio we only spawn 2 netdevs if the backup bit is set
>>> which implies a specific use case. When the bit isn't set we are only
>>> spawning one device, and as a result we can get much better
>>> performance out of the resultant combination of devices, and can
>>> expose functionality such as in-driver XDP in virtio.
>>
>> I have experimented with toggling IFF_NOQUEUE and/or LLTX on the netvsc
>> device when doing passthrough. It didn't help performance much and there
>> are corner cases that make it painful, like what if qdisc was placed
>> by user on the netvsc device?  Should the qdisc then be moved back
>> and forth to the VF device when it arrives or is removed?
>>
>> As far as XDP support, it is on the plan to support XDP on the netvsc
>> device since the receive buffers do have to be copied. But there has
>> been no customer demand for it; and the VF model on Azure has limitations
>> which make a transparent XDP model pretty much impossible.
>
> Jiri, does that answer your question? Even though there is some
> similarity, the needs of netvsc and virtio appear significantly
> different.
>
> We do not yet know whether other PV devices will be virtio-like
> or netvsc-like.
>
> Do you agree?

Michael,

At this point can we agree to go with 3 netdev model for virtio and as 
this is
not compatible with netvsc's 2 netdev model,  the scope for any common code
between virtio and netvsc is very limited.

Should i submit a v5 version of this patch series with some minor 
fixes?  Or can
you pull in this series and i can submit patches on top of that?

Thanks
Sridhar