Unikraft virtio_ring.c Error Reproduction

This repository reproduces the unikraft/unikraft#853 virtio_ring.c error:

Waiting for connections!
Writing!
Half-time break!
Break over!
[    3.335716] ERR:  [libkvmvirtio] <virtio_ring.c @  348> Available descriptor:0, Request2
[    3.338315] ERR:  [libkvmvirtio] <virtio_ring.c @  348> Available descriptor:0, Request2
Written everything!
Client served!
Waiting for connections!

This is a minimal server-client setup. The server transmits 128 MiB to the client. In the middle of the transmission, the server makes a short break. Then the transmission continues, resulting in the error.

The error relies on a race condition. It may trigger more or less often depending on:

• The size of the unikernel (e.g. adding musl to the binary makes it more frequent).
• The size of the buffer (bigger transmissions tend to cause errors more reliable).
• The pause (without pauses inbetween transmissions, the error won't show up).
• The speed of the QEMU network device (user mode emulation does not show the error, TAP devices do).

How to Reproduce

0. Make sure to check out the submodules for Unikraft and lwIP sources.

   git submodule init

1. Setup the TAP device.

   sudo ./setup_tap.sh

2. Start the server using Unikraft.

   ./run_server.sh

3. From another terminal, start the client.

   ./run_client.sh