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

[David Miller <davem@davemloft.net>]

From: Tom Herbert <tom@herbertland.com>
Date: Thu, 31 Mar 2016 17:18:48 -0400

> On Tue, Mar 29, 2016 at 6:40 PM, Guus Sliepen <guus@tinc-vpn.org> wrote:
>> 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.