CAmkES VM: Cross VM Connectors

This tutorial provides an introduction to using the cross virtual machine (VM) connector mechanisms provided by seL4 and Camkes in order to connect processes in a guest Linux instance to Camkes components.


  1. Set up your machine.
  2. Camkes VM


By the end of this tutorial, you should be able to:

  • Configure processes in a Linux guest VM to communicate with CAmkES components


In order to connect guest Linux instances to CAmkES components, three additional kernel modules must be installed in the guest. These modules are included in the root filesystem by default:

  • dataport: facilitates setting up shared memory between the guest and CAmkES components.
  • consumes_event: allows a process in the guest to wait or poll for an event sent by a CAmkES component.
  • emits_event: allows a process to emit an event to a CAmkES component.

Each type of module can be statically assigned to one or more file descriptors to associate that file descriptor with a specific instance of an interface. ioctl can then be used to manipulate that file descriptor and use the module.


Dataports are the mechanism which allows guests and components to share memory. The dataport initialisation process is as follows:

  • The guest process uses ioctl on on the file associated with the dataport and specify a page-aligned size for the shared memory.
  • The dataport kernel module in the guest then allocates a page-aligned buffer of the requested size, and makes a hypercall to the VMM, with the guest physical address and id of the data port. The ID is derived from the file on which ioctl was called.
  • The virtual machine manager (VMM) then modifies the guest’s address space, creating the shared memory region. between a camkes component and the guest.
  • Linux processes can then map this memory into their own address space by calling mmap on the file associated with the dataport.

Emitting Events

Guest processes can emit events by using ioctl on files associated with the event interface. This results in the emits_event kernel module in the guest making a making a hypercall to the VMM, which triggers the event and resumes the guest.

Consuming Events

Linux process can wait or poll for an event by calling poll on the file associated with that event, using the timeout argument to specify whether or not it should block. The event it polls for is POLLIN. When the VMM receives an event destined for the guest, it places the event id in some memory shared between the VMM and the consumes_event kernel module, and then injects an interrupt into the guest. The consumes_event kernel module is registered to handle this interrupt, which reads the event ID from shared memory, and wakes a thread blocked on the corresponding event file. If no threads are blocked on the file, some state is set in the module such that the next time a process waits on that file, it returns immediately and clears the state, mimicking the behaviour of notifications.


In this tutorial you will create a program that runs in the guest, and sends a string to a CAmkES component to output. To achieve this, the guest program will write a string to a shared buffer between itself and a CAmkES component. When its ready for the string to be printed, it will emit an event, received by the CAmkES component. The CAmkES component will print the string, then send an event to the guest process so the guest knows it’s safe to send a new string.

Add modules to the guest

There is a library in projects/camkes/vm-linux/camkes-linux-artifacts/camkes-linux-apps/camkes-connector-apps/libs containing Linux system call wrappers, and some utility programs in projects/camkes/vm-linux/camkes-linux-artifacts/camkes-linux-apps/camkes-connector-apps/pkgs/{dataport,consumes_event,emits_event} which initialize and interact with cross VM connections. To build and use these modules in your rootfs the vm-linux project provides an overlay target you can use.

Exercise First add the dataport, consumes_event and emits_event kernel modules to the rootfs in the guest.

Start by replacing the line:

AddToFileServer("rootfs.cpio" ${default_rootfs_file})

in the target applications CMakeLists.txt file with the following:

set(CAmkESVMDefaultBuildrootOverlay ON CACHE BOOL "" FORCE)
AddOverlayDirToRootfs(default_buildroot_overlay ${default_rootfs_file} "buildroot" "rootfs_install"
    rootfs_file rootfs_target)
AddToFileServer("rootfs.cpio" ${rootfs_file})

Define interfaces in the VMM

Exercise Update the CAmkES file, crossvm_tutorial.camkes by replacing the Init0 component definition:

component Init0 {

with the following definition:

component Init0 {

  // this is the data port for shared memory between the component and guest process
  dataport Buf(4096) data;
  // this event tells the component that there is data ready to print
  emits DoPrint do_print;
  // this event tells the guest process that priting is complete
  consumes DonePrinting done_printing;
  // this mutex protects access to shared state between the VMM and the guest Linux
  has mutex cross_vm_event_mutex;

These interfaces will eventually be made visible to processes running in the guest linux. The mutex is used to protect access to shared state between the VMM and guest.

Define the component interface

Exercise Define the print server component by adding the following to the crossvm_tutorial.camkes file, after the Init0 definition:

component PrintServer {
  dataport Buf(4096) data;
  consumes DoPrint do_print;
  emits DonePrinting done_printing;

Instantiate the print server

Exercise Replace the composition definition:

    composition {

with the following:

    composition {

        component PrintServer print_server;
        connection seL4Notification conn_do_print(from vm0.do_print,
                                                 to print_server.do_print);
        connection seL4Notification conn_done_printing(from print_server.done_printing,
                                                      to vm0.done_printing);

        connection seL4SharedDataWithCaps conn_data(from,

The seL4SharedDataWithCaps connector is a dataport connector much like seL4SharedData. However, the to side of the connection also receives access to the capabilities to the frames backing the dataport, which is required for cross VM dataports, as the VMM must be able to establish shared memory at runtime by inserting new mappings into the guest’s address space.

Exercise Interfaces connected with seL4SharedDataWithCaps must be configured with an integer specifying the ID and size of the dataport. Do this now by modifying crossvm_tutorial.camkes with the following two lines in the configuration section:

    configuration {
        // Add the following 2 lines:
        vm0.data_id = 1; // ids must be contiguous, starting from 1
        vm0.data_size = 4096;

Implement the print server

Exercise Add the file components/print_server.c with the following contents:

#include <camkes.h>
#include <stdio.h>

int run(void) {

    while (1) {
        // wait for the next event

        printf("%s\n", (char*)data);

        // signal that we are done printing

    return 0;

This provides a very simple component definition that loops forever, printing a string from shared memory whenever an event is received then emitting an event. The example code assumes that the shared buffer will contain a valid, null-terminated c string, which is not something you should do in practice.

Implement the VMM side of the connection

Create another c file that tells the VMM about the cross VM connections. This file must define 3 functions which initialize each type of cross vm interface:

  • cross_vm_dataports_init
  • cross_vm_emits_events_init
  • cross_vm_consumes_events_init

Exercise Add a file src/cross_vm.c with the following contents:

#include <sel4/sel4.h>
#include <camkes.h>
#include <camkes_mutex.h>
#include <camkes_consumes_event.h>
#include <camkes_emits_event.h>
#include <dataport_caps.h>
#include <cross_vm_consumes_event.h>
#include <cross_vm_emits_event.h>
#include <cross_vm_dataport.h>
#include <vmm/vmm.h>
#include <vspace/vspace.h>

// this is defined in the dataport's glue code
extern dataport_caps_handle_t data_handle;

// Array of dataport handles at positions corresponding to handle ids from spec
static dataport_caps_handle_t *dataports[] = {
    NULL, // entry 0 is NULL so ids correspond with indices

// Array of consumed event callbacks and ids
static camkes_consumes_event_t consumed_events[] = {
    { .id = 1, .reg_callback = done_printing_reg_callback },

// Array of emitted event emit functions
static camkes_emit_fn emitted_events[] = {
    NULL,   // entry 0 is NULL so ids correspond with indices

// mutex to protect shared event context
static camkes_mutex_t cross_vm_event_mutex = (camkes_mutex_t) {
    .lock = cross_vm_event_mutex_lock,
    .unlock = cross_vm_event_mutex_unlock,

int cross_vm_dataports_init(vmm_t *vmm) {
    return cross_vm_dataports_init_common(vmm, dataports, sizeof(dataports)/sizeof(dataports[0]));

int cross_vm_emits_events_init(vmm_t *vmm) {
    return cross_vm_emits_events_init_common(vmm, emitted_events,

int cross_vm_consumes_events_init(vmm_t *vmm, vspace_t *vspace, seL4_Word irq_badge) {
    return cross_vm_consumes_events_init_common(vmm, vspace, &cross_vm_event_mutex,
            consumed_events, sizeof(consumed_events)/sizeof(consumed_events[0]), irq_badge);

Update the build system

Exercise Make the following changes in CMakeLists.txt by firstly replacing the declaration of Init0:


with the following declaration:

# Retrieve Init0 cross vm src files
file(GLOB init0_extra src/*.c)
# Declare VM component: Init0
    EXTRA_SOURCES ${init0_extra}
    EXTRA_LIBS crossvm

Also add a declaration for a PrintServer component:

# Declare the CAmkES PrintServer component
DeclareCAmkESComponent(PrintServer SOURCES components/print_server.c)

This extends the definition of the Init component with the cross_vm connector source and the crossvm library, and defines the new CAmkES component PrintServer.

Add interfaces to the Guest

Exercise Create the following camkes_init shell script that is executed as Linux is initialized:

# Initialises linux-side of cross vm connections.

# Dataport sizes must match those in the camkes spec.
# For each argument to dataport_init, the nth pair
# corresponds to the dataport with id n.
dataport_init /dev/camkes_data 4096

# The nth argument to event_init corresponds to the
# event with id n according to the camkes vmm.
consumes_event_init /dev/camkes_done_printing
emits_event_init /dev/camkes_do_print

Each of these commands creates device nodes associated with a particular Linux kernel module supporting cross VM communication. Each command takes a list of device nodes to create, which must correspond to the IDs assigned to interfaces in crossvm_tutorial.camkes and cross_vm.c. The dataport_init command must also be passed the size of each dataport.

These changes will cause device nodes to be created which correspond to the interfaces you added to the VMM component.

Create a process

Now make a process that uses the device nodes to communicate with the print server.

Exercise First create a new directory:

mkdir -p pkgs/print_client

with the following file pkgs/print_client/print_client.c:

#include <string.h>
#include <assert.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>

#include "dataport.h"
#include "consumes_event.h"
#include "emits_event.h"

int main(int argc, char *argv[]) {

    int data_fd = open("/dev/camkes_data", O_RDWR);
    assert(data_fd >= 0);

    int do_print_fd = open("/dev/camkes_do_print", O_RDWR);
    assert(do_print_fd >= 0);

    int done_printing_fd = open("/dev/camkes_done_printing", O_RDWR);
    assert(done_printing_fd >= 0);

    char *data = (char*)dataport_mmap(data_fd);
    assert(data != MAP_FAILED);

    ssize_t dataport_size = dataport_get_size(data_fd);
    assert(dataport_size > 0);

    for (int i = 1; i < argc; i++) {
        strncpy(data, argv[i], dataport_size);


    return 0;

This program prints each of its arguments on a separate line, by sending each argument to the print server one at a time.

Exercise create pkgs/print_client/CMakeLists.txt for our client program:

cmake_minimum_required(VERSION 3.8.2)

project(print_client C)

add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/../../../projects/camkes/vm-linux/camkes-linux-artifacts/camkes-linux-apps/camkes-connector-apps/libs/camkes camkes)

add_executable(print_client print_client.c)
target_link_libraries(print_client camkeslinux)

Exercise Update our the VM apps CMakeLists.txt. Below the line:

AddToFileServer("bzimage" ${decompressed_kernel} DEPENDS extract_linux_kernel)

add the ExternalProject declaration to include the print application:

# Get Custom toolchain for 32 bit Linux
FindCustomPollyToolchain(LINUX_32BIT_TOOLCHAIN "linux-gcc-32bit-pic")
# Declare our print server app external project
    SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/pkgs/print_client
    BINARY_DIR ${CMAKE_BINARY_DIR}/print_client-app
    STAMP_DIR ${CMAKE_CURRENT_BINARY_DIR}/print_client-app-stamp
# Add the print client app to our overlay ('default_buildroot_overlay')
AddExternalProjFilesToOverlay(print_client-app ${CMAKE_BINARY_DIR}/print_client-app default_buildroot_overlay "usr/sbin"
    FILES print_client)```

Directly below this we also want to add our `camkes_init` script into the overlay. We place this into the VMs `init.d` directory so
the script is run on start up:

AddFileToOverlayDir("S90camkes_init" ${CMAKE_CURRENT_LIST_DIR}/camkes_init "etc/init.d" default_buildroot_overlay)

That’s it. Build and run the system, and you should see the following output:

Creating dataport node /dev/camkes_data
Allocating 4096 bytes for /dev/camkes_data
Creating consuming event node /dev/camkes_done_printing
Creating emitting event node /dev/camkes_do_print

Welcome to Buildroot
buildroot login: root
# print_client hello world
[   12.730073] dataport received mmap for minor 1