Re: [Qemu-devel] Contribution - L2TPv3 transport

[Stefan Hajnoczi <stefanha@gmail.com>]

On Fri, Feb 28, 2014 at 08:28:11AM +0000, Anton Ivanov (antivano) wrote:
> 3. Qemu to communicate with the local host, remote vms, network devices,
> etc at speeds which for a number of use cases exceed the speed of the
> legacy tap driver.

This surprises me.  It's odd that tap performs significantly worse.

I guess you have two options for using kernel L2TPv3 support:

1. Use a bridge and tap to forward between the L2TPv3 interface and the
   QEMU.

2. Use macvtap on top of the L2TPv3 interface (if possible).

Option #2 should be faster.

Now about the tap userspace ABI, is the performance bottleneck that the
read(2) system call only receives one packet at a time?  The tap file
descriptor is not a socket so recvmmsg(2) cannot be used on it directly.

I have wondered in the past whether something like packet mmap would be
possible on a tap device.  At that point userspace just waits for
notifications and the actual packets don't require any syscalls.

> Our suggestion would be that this over time obsoletes the UDP variety of
> the "socket" driver.

Yes, thank you!  L2TPv3 looks like a good replacement for the "socket"
driver.

Will review and respond to your patch in detail.

Stefan