From: Tom Herbert <redacted>
Date: Thu, 31 Mar 2016 17:18:48 -0400

On Tue, Mar 29, 2016 at 6:40 PM, Guus Sliepen [off-list ref] wrote:

quoted

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?

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.