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

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Date:	Wed, 30 Mar 2016 00:40:43 +0200
From:	Guus Sliepen <guus@...c-vpn.org>
To:	netdev@...r.kernel.org
Subject: Best way to reduce system call overhead for tun device I/O?

I'm trying to reduce system call overhead when reading/writing to/from a
tun device in userspace. For sockets, one can use sendmmsg()/recvmmsg(),
but a tun fd is not a socket fd, so this doesn't work. I'm see several
options to allow userspace to read/write multiple packets with one
syscall:

- Implement a TX/RX ring buffer that is mmap()ed, like with AF_PACKET
  sockets.

- Implement a ioctl() to emulate sendmmsg()/recvmmsg().

- Add a flag that can be set using TUNSETIFF that makes regular
  read()/write() calls handle multiple packets in one go.

- Expose a socket fd to userspace, so regular sendmmsg()/recvmmsg() can
  be used. There is tun_get_socket() which is used internally in the
  kernel, but this is not exposed to userspace, and doesn't look trivial
  to do either.

What would be the right way to do this?

-- 
Met vriendelijke groet / with kind regards,
     Guus Sliepen <guus@...c-vpn.org>