layout: post title: “X86 Linux Boot Status on gem5-19” author: Ayaz Akram date: 2020-03-09 categories: project


The frequency of changes pushed to gem5 is increasing with time. This makes it important to have an up-to-date idea of what is working with gem5 and what is not. The booting of the Linux kernel is a very important benchmark to ascertain the working status of gem5, considering that gem5 is a full-system simulator which should be able to simulate a modern operating system. However, the state of support of latest Linux kernel versions on gem5 is hard to discover, and, previously available Linux kernels or configuration files on gem5 website are quite old. gem5-19 has also been released recently.

We thereby ran tests to discover where gem5-19 stands in terms of its ability to boot the latest releases of the Linux kernel. In this post, we will discuss the results of these tests.

Configuration Space

The possible configuration space when simulating a Linux boot on gem5 is large. To test gem5-19, we evaluated multiple configurations, taking into account five Linux kernels, four CPU models, two memory systems, two Linux boot types, and four CPU core counts

We conducted these tests using the X86 ISA. Our methodology should extend easily to other ISAs as well, but we haven't run these tests, yet. We welcome other contributors to run these tests!

Following are the details of each of these:

Linux Kernel

We evaluated gem5 using the five latest LTS (long term support) kernels shown below. We plan on continuing to test the LTS kernel releases and the most recent kernel on each gem5 release.

  • v4.4.186 (released on 2016-01-10)
  • v4.9.186 (released on 2016-12-11)
  • v4.14.134 (released on 2017-11-12)
  • v4.19.83 (released on 2018-10-22)
  • v5.4 (released on 2019-11-24)

CPU Model

We used four CPU models supported in gem5:

  • kvmCPU: A CPU that does not do any timing simulation, but rather uses actual hardware to run the simulated code. It is mainly used for fast forwarding.
  • AtomicSimpleCPU: This CPU also does not do any timing simulation and uses atomic memory accesses. It is mostly used for fast forwarding and cache warming.
  • TimingSimpleCPU: This is a single cycle CPU model except for memory operations (it uses timing memory accesses).
  • O3CPU: This is a detailed and highly configurable out of order CPU model (it does timing simulation for both CPU and memory).

Memory System

There are two main memory systems supported in gem5 (which are used in these tests):

  • Classic: The Classic memory system is fast and easily configurable and supports atomic accesses, but lacks cache coherence fidelity and flexibility (it models a simplified coherence protocol).
  • Ruby: The Ruby memory system models a detailed cache memory with detailed cache coherence protocols. However, it does not support atomic accesses and is slower compared to the classic memory system.

Boot Type

Boot type refers to the kind of process which will take over once the kernel loads. We use two different options:

  • init: A custom init script which exits the system using the m5 exit instruction.
  • systemd: Systemd is the default init system that makes the system ready for use by initializing different services and managing user processes.


We used gem5art (library for artifacts, reproducibility, and testing) to perform these experiments. gem5art helps us to conduct gem5 experiments in a more structured and reproducible way. We will, however, defer the detailed discussion on gem5art for a future blog post. The gem5 configuration scripts used to run these experiments are available in the gem5art repo and the details of how these experiments were run using gem5art can be found in the gem5art boot tutorial. The disk image and Linux kernel binaries we used are available from the following links (warning: the sizes of these files range from few MBs to 2GB):

Linux Booting Status

Figure 1 and 2 show the results of these experiments with the classic memory system for the init and systemd boot types respectively. Figure 3 and 4 show the results of these experiments for the ruby memory system for the init and systemd boot types respectively. All possible status outputs (shown in the figures below) are defined as follows:

  • timeout: experiment did not finish in a reasonable amount of time (8 hours: this time was chosen as we found similar successful cases did not exceed this limit on the same host machine).
  • not-supported: cases which are not yet supported in gem5.
  • success: cases where Linux booted successfully.
  • sim-crash: cases where gem5 crashed.
  • kernel-panic: cases where kernel went into panic during simulation.

When using a classic memory system, KVM and Atomic CPU models always work. TimingSimple CPU always works for a single core, but fails to boot the kernel for multiple CPU cores. The O3 CPU model fails to simulate kernel booting in most of the cases (the only success is init boot type with two Linux kernel versions).