It is midterm evaluation time, so half the time is up and I want to write about the changes I bring into the RTEMS project. This post will explain the patch I created for midterm.

Overview

I brought one major change to the project, the split of the i386 architecture dependent code into a part in score and another part in libcpu. Compared to this the virtualization layer consisting of two header files, one for CPU dependent functionality and on for board dependent, is rather small. The introduction of the new BSP called virtPOK is a bigger change, but still, it only affects this project and not all building upon the i386 architecture.

i386 split

i386 has several instructions (hlt, cli, sti) which are not allowed in a virtualized environment, as they affect other virtual machines. Functions in cpukit/score/cpu/i386/ are using these instruction and therefore need to be moved. However, these functions must remain in the native version of an RTEMS i386 binary, but must be replaced with different implementations in a virtual compilation.

RTEMS is providing with libcpu a place where CPUs with special features of the same architecture can be specified. That is the place where the native and virtual implementations of the mentioned functions need to go. I introduced two new CPU models for the i386 architecture – native and virtual. Native is used by the pc386-BSPs and should work like before. Virtual is the new kid in the block and is only used by the virtPok BSP.

Configure.ac is controlling the choice of the bsp. As the config file for a BSP specifies the CPU model, the virtPok.cfg is specifying virt-pok as CPU. Configure.ac checks for this CPU model and sets a conditional if this model is used. The conditional controls the path of execution in the corresponding Makefile.am, which in turn chooses the virtual or native directory.

Virtualization layer

The virtualization layer consists of two parts: One for the CPU and one for the BSP. My intention for the layer is to provide on BSP using the functionality of the layer and then the layer must be implemented by the host/hypervisor and provided as a library to RTEMS.

The CPU part needs to be implemented in RTEMS once for each architecture. The BSP currently developed for i386 might be reusable on other architectures, as the host cares for architecture dependent parts.

Both header files virtLayerBSP.h and virtLayerCPU.h can be found in c/src/lib/libbsp/i386/virtPok/include/ .

Virtual BSP

The virtual BSP for i386 is called virtPok and is currently able to run the hello world sample virtualized on POK. The BSP mimics the interface between RTEMS and the host/hypervisor, by forwarding the RTEMS function calls to the virtualization layer, which is implemented by the host and provided as a library to the BSP. Currently implemented is a console driver to run the hello world sample. A clock driver will be the next step but this needs changes in POK first.

Putting it all together

To rebuild the current work, you can either checkout my virt-bsp branch from github

Or you can apply the patches from the rtems-devel mailing list to the current RTEMS git head.

Then you need to follow my explanations from the Hello World blog post.

Unfortunately, you need to build RTEMS with make -j1 as parallel compilation can lead to undefined reference errors.