netdev - Re: [RFC PATCH 2/3] netdev: kernel-only IFF

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Message-ID: <CADGSJ211kA5ktADbwv2f4ma_ooBmYW4r7YwzGD2G_Gw5+wv5rw@mail.gmail.com>
Date:   Mon, 9 Apr 2018 15:07:26 -0700
From:   Siwei Liu <loseweigh@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     David Miller <davem@...emloft.net>,
        David Ahern <dsahern@...il.com>, Jiri Pirko <jiri@...nulli.us>,
        si-wei liu <si-wei.liu@...cle.com>,
        "Michael S. Tsirkin" <mst@...hat.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>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        Netdev <netdev@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        virtio-dev@...ts.oasis-open.org
Subject: Re: [RFC PATCH 2/3] netdev: kernel-only IFF_HIDDEN netdevice

On Fri, Apr 6, 2018 at 8:19 PM, Andrew Lunn <andrew@...n.ch> wrote:
> Hi Siwei
>
>> I think everyone seems to agree not to fiddle with the ":" prefix, but
>> rather have a new class of network subsystem under /sys/class thus a
>> separate device namespace e.g. /sys/class/net-kernel for those
>> auto-managed lower netdevs is needed.
>
> How do you get a device into this new class? I don't know the Linux
> driver model too well, but to get a device out of one class and into
> another, i think you need to device_del(dev). modify dev->class and
> then device_add(dev). However, device_add() says you are not allowed
> to do this.

No, implementation wise I'd avoid changing the class on the fly. What
I'm looking to is a means to add a secondary class or class aliasing
mechanism for netdevs that allows mapping for a kernel device
namespace (/class/net-kernel) to userspace (/class/net). Imagine
creating symlinks between these two namespaces as an analogy. All
userspace visible netdevs today will have both a kernel name and a
userspace visible name, having one (/class/net) referecing the other
(/class/net-kernel) in its own namespace. The newly introduced
IFF_AUTO_MANAGED device will have a kernel name only
(/class/net-kernel). As a result, the existing applications using
/class/net don't break, while we're adding the kernel namespace that
allows IFF_AUTO_MANAGED devices which will not be exposed to userspace
at all.

As this requires changing the internals of driver model core it's a
rather big hammer approach I'd think. If there exists a better
implementation than this to allow adding a separate layer of in-kernel
device namespace, I'd more than welcome to hear about.

>
> And i don't even see how this helps. Are you also not going to call
> list_netdevice()? Are you going to add some other list for these
> devices in a different class?

list_netdevice() is still called. I think with the current RFC patch,
I've added two lists for netdevs under the kernel namespace:
dev_cmpl_list and name_cmpl_hlist. As a result of that, all userspace
netdevs get registered will be added to two types of lists: the
userspace list for e.g. dev_list, and also the kernelspace list e.g.
dev_cmpl_list (I can rename it to something more accurate). The
IFF_AUTO_MANAGED device will be only added to kernelspace list e.g.
dev_cmpl_list.

Hope all your questions are answered.

Thanks,
-Siwei

>
>    Andrew