netdev - Re: Best way to reduce system call overhead for tun device I/O?

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Message-ID: <20160331163932.3617e7cd@xeon-e3>
Date:	Thu, 31 Mar 2016 16:39:32 -0700
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Guus Sliepen <guus@...c-vpn.org>
Cc:	David Miller <davem@...emloft.net>, tom@...bertland.com,
	netdev@...r.kernel.org
Subject: Re: Best way to reduce system call overhead for tun device I/O?

On Fri, 1 Apr 2016 00:28:57 +0200
Guus Sliepen <guus@...c-vpn.org> wrote:

> On Thu, Mar 31, 2016 at 05:20:50PM -0400, David Miller wrote:
> 
> > >> I'm trying to reduce system call overhead when reading/writing to/from a
> > >> tun device in userspace. [...] What would be the right way to do this?
> > >>
> > > Personally I think tun could benefit greatly if it were implemented as
> > > a socket instead of character interface. One thing that could be much
> > > better is sending/receiving of meta data attached to skbuf. For
> > > instance GSO data could be in ancillary data in a socket instead of
> > > inline with packet data as tun seems to be doing now.
> > 
> > Agreed.
> 
> Ok. So how should the userspace API work? Creating an AF_PACKET socket
> and then using a tun ioctl to create a tun interface and bind it to the
> socket?
> 
> int fd = socket(AF_PACKET, ...)
> struct ifreq ifr = {...};
> ioctl(fd, TUNSETIFF, &ifr);
> 

Rather than bodge AF_PACKET onto TUN, why not just create a new device type
and control it from something modern like netlink.