Linux Virtualization - Apr 2018 - [RFC PATCH 2/3] netdev: kernel-only IFF_HIDDEN netdevice

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.

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?

   Andrew

On Fri, Apr 6, 2018 at 8:19 PM, Andrew Lunn <andrew at lunn.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

> 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.
My gut feeling is this whole scheme will not fly. You really should be
talking to GregKH.

Anyway, please remember that IFF_AUTO_MANAGED will need to be dynamic.
A device can start out as a normal device, and will change to being
automatic later, when the user on top of it probes.

	Andrew