netdev - Re: [PATCH net] failover: eliminate callback hell

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Date:   Thu, 7 Jun 2018 18:41:31 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        Jiri Pirko <jiri@...nulli.us>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>,
        Stephen Hemminger <sthemmin@...rosoft.com>
Subject: Re: [PATCH net] failover: eliminate callback hell

On Thu, Jun 07, 2018 at 07:51:12AM -0700, Stephen Hemminger wrote:
> On Thu, 7 Jun 2018 07:17:51 -0700
> Alexander Duyck <alexander.duyck@...il.com> wrote:
> 
> > On Wed, Jun 6, 2018 at 3:25 PM, Stephen Hemminger
> > <stephen@...workplumber.org> wrote:
> > > On Wed, 6 Jun 2018 14:54:04 -0700
> > > "Samudrala, Sridhar" <sridhar.samudrala@...el.com> wrote:
> > >  
> > >> On 6/6/2018 2:24 PM, Stephen Hemminger wrote:  
> > >> > On Wed, 6 Jun 2018 15:30:27 +0300
> > >> > "Michael S. Tsirkin" <mst@...hat.com> wrote:
> > >> >  
> > >> >> On Wed, Jun 06, 2018 at 09:25:12AM +0200, Jiri Pirko wrote:  
> > >> >>> Tue, Jun 05, 2018 at 05:42:31AM CEST, stephen@...workplumber.org wrote:  
> > >> >>>> The net failover should be a simple library, not a virtual
> > >> >>>> object with function callbacks (see callback hell).  
> > >> >>> Why just a library? It should do a common things. I think it should be a
> > >> >>> virtual object. Looks like your patch again splits the common
> > >> >>> functionality into multiple drivers. That is kind of backwards attitude.
> > >> >>> I don't get it. We should rather focus on fixing the mess the
> > >> >>> introduction of netvsc-bonding caused and switch netvsc to 3-netdev
> > >> >>> model.  
> > >> >> So it seems that at least one benefit for netvsc would be better
> > >> >> handling of renames.
> > >> >>
> > >> >> Question is how can this change to 3-netdev happen?  Stephen is
> > >> >> concerned about risk of breaking some userspace.
> > >> >>
> > >> >> Stephen, this seems to be the usecase that IFF_HIDDEN was trying to
> > >> >> address, and you said then "why not use existing network namespaces
> > >> >> rather than inventing a new abstraction". So how about it then? Do you
> > >> >> want to find a way to use namespaces to hide the PV device for netvsc
> > >> >> compatibility?
> > >> >>  
> > >> > Netvsc can't work with 3 dev model. MS has worked with enough distro's and
> > >> > startups that all demand eth0 always be present. And VF may come and go.
> > >> > After this history, there is a strong motivation not to change how kernel
> > >> > behaves. Switching to 3 device model would be perceived as breaking
> > >> > existing userspace.  
> > >>
> > >> I think it should be possible for netvsc to work with 3 dev model if the only
> > >> requirement is that eth0 will always be present. With net_failover, you will
> > >> see eth0 and eth0nsby OR with older distros eth0 and eth1.  It may be an issue
> > >> if somehow there is userspace requirement that there can be only 2 netdevs, not 3
> > >> when VF is plugged.
> > >>
> > >> eth0 will be the net_failover device and eth0nsby/eth1 will be the netvsc device
> > >> and the IP address gets configured on eth0. Will this be an issue?  
> > >
> > > DPDK drivers in 18.05 depend on 2 device model. Yes it is a bit of mess
> > > but that is the way it is.  
> > 
> > Why would DPDK care what we do in the kernel? Isn't it just slapping
> > vfio-pci on the netdevs it sees?
> 
> Alex, you are correct for Intel devices; but DPDK on Azure is not Intel based.,.
> The DPDK support uses:
>  * Mellanox MLX5 which uses the Infinband hooks to do DMA directly to
>    userspace. This means VF netdev device must exist and be visible.
>  * Slow path using kernel netvsc device, TAP and BPF to get exception
>    path packets to userspace.
>  * A autodiscovery mechanism that to set all this up that relies on
>    2 device model and sysfs.

Could you describe what does it look for exactly? What will break if
instead of MLX5 being a child of the PV, it's a child of the failover
device?

> In this version, there is no VFIO-PCI. And also Hyper-V does not have virtual
> IOMMU so VFIO will not work there at all.
>