Re: [RFC PATCH net-next v2 2/2] virtio_net: Extend virtio to use VF datapath when available

[Siwei Liu <loseweigh@gmail.com>]

On Mon, Jan 22, 2018 at 1:41 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Mon, Jan 22, 2018 at 12:27:14PM -0800, Siwei Liu wrote:
>> First off, as mentioned in another thread, the model of stacking up
>> virt-bond functionality over virtio seems a wrong direction to me.
>> Essentially the migration process would need to carry over all guest
>> side configurations previously done on the VF/PT and get them moved to
>> the new device being it virtio or VF/PT.
>
> I might be wrong but I don't see why we should worry about this usecase.
> Whoever has a bond configured already has working config for migration.
> We are trying to help people who don't, not convert existig users.

That has been placed in the view of cloud providers that the imported
images from the store must be able to run unmodified thus no
additional setup script is allowed (just as Stephen mentioned in
another mail). Cloud users don't care about live migration themselves
but the providers are required to implement such automation mechanism
to make this process transparent if at all possible. The user does not
care about the device underneath being VF or not, but they do care
about consistency all across and the resulting performance
acceleration in making VF the prefered datapath. It is not quite
peculiar user cases but IMHO *any* approach proposed for live
migration should be able to persist the state including network config
e.g. as simple as MTU. Actually this requirement has nothing to do
with virtio but our target users are live migration agnostic, being it
tracking DMA through dirty pages, using virtio as the helper, or
whatsoever, the goal of persisting configs across remains same.

>
>> Without the help of a new
>> upper layer bond driver that enslaves virtio and VF/PT devices
>> underneath, virtio will be overloaded with too much specifics being a
>> VF/PT backup in the future.
>
> So this paragraph already includes at least two conflicting
> proposals. On the one hand you want a separate device for
> the virtual bond, on the other you are saying a separate
> driver.

Just to be crystal clear: separate virtual bond device (netdev ops,
not necessarily bus device) for VM migration specifically with a
separate driver.

>
> Further, the reason to have a separate *driver* was that
> some people wanted to share code with netvsc - and that
> one does not create a separate device, which you can't
> change without breaking existing configs.

I'm not sure I understand this statement. netvsc is already another
netdev being created than the enslaved VF netdev, why it bothers? In
the Azure case, the stock image to be imported does not bind to a
specific driver but only MAC address. And people just deal with the
new virt-bond netdev rather than the underlying virtio and VF. And
both these two underlying netdevs should be made invisible to prevent
userspace script from getting them misconfigured IMHO.

A separate driver was for code sharing for sure, only just netvsc but
could be other para-virtual devices floating around: any PV can serve
as the side channel and the backup path for VF/PT. Once we get the new
driver working atop virtio we may define ops and/or protocol needed to
talk to various other PV frontend that may implement the side channel
of its own for datapath switching (e.g. virtio is one of them, Xen PV
frontend can be another). I just don't like to limit the function to
virtio only and we have to duplicate code then it starts to scatter
around all over the places.

I understand right now we start it as simple so it may just be fine
that the initial development activities center around virtio. However,
from cloud provider/vendor perspective I don't see the proposed scheme
limits to virtio only. Any other PV driver which has the plan to
support the same scheme can benefit. The point is that we shouldn't be
limiting the scheme to virtio specifics so early which is hard to have
it promoted to a common driver once we get there.

>
> So some people want a fully userspace-configurable switchdev, and that
> already exists at some level, and maybe it makes sense to add more
> features for performance.
>
> But the point was that some host configurations are very simple,
> and it probably makes sense to pass this information to the guest
> and have guest act on it directly. Let's not conflate the two.

It may be fine to push some of the configurations from host but that
perhaps doesn't cover all the cases: how is it possible for the host
to save all network states and configs done by the guest before
migration. Some of the configs might come from future guest which is
unknown to host. Anyhow the bottom line is that the guest must be able
to act on those configuration request changes automatically without
involving users intervention.

Regards,
-Siwei

>
> --
> MST