website: Add a blog post on linux boot tests with gem5-19
Change-Id: I02bced3f985a6d84e0f6e8c2ff7c6f348f88388f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/26523
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
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+---
+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](https://www.gem5.org/project/2020/02/25/gem5-19.html) 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.
+
+## gem5art
+
+We used [gem5art](https://gem5art.readthedocs.io/en/latest/index.html) (library for **a**rtifacts, **r**eproducibility, and **t**esting) 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](https://github.com/darchr/gem5art/tree/master/docs/gem5-configs/configs-boot-tests/) and the details of how these experiments were run using gem5art can be found in the [gem5art boot tutorial](https://gem5art.readthedocs.io/en/latest/tutorials/boot-tutorial.html).
+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):
+
+- [disk image](http://dist.gem5.org/images/x86/ubuntu-18-04/base.img) (**Note:** /root/.bashrc in this disk image contains `m5 exit`, which will make the guest terminate the simulation as soon as it boots)
+- [vmlinux-4.4.186](http://dist.gem5.org/kernels/x86/static/vmlinux-4.4.189)
+- [vmlinux-4.9.186](http://dist.gem5.org/kernels/x86/static/vmlinux-4.9.186)
+- [vmlinux-4.14.134](http://dist.gem5.org/kernels/x86/static/vmlinux-4.14.134)
+- [vmlinux-4.19.83](http://dist.gem5.org/kernels/x86/static/vmlinux-4.19.83)
+- [vmlinux-5.4](http://dist.gem5.org/kernels/x86/static/vmlinux-5.4)
+
+## 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).
+
+
+<br>
+*Figure 1: Linux boot status for classic memory system and init boot*
+
+
+
+<br>
+*Figure 2: Linux boot status for classic memory system and systemd boot*
+
+As shown in Figure 3 and 4, for Ruby memory system, KVM and Atomic CPU models seem to work except for a couple of cases where even the KVM CPU model times out.
+TimingSimple CPU works up to 2 cores, but fails for 4 and 8 cores.
+The O3 CPU model fails to simulate Linux booting or times out in all cases.
+
+
+<br>
+*Figure 3: Linux boot status for ruby memory system and init boot*
+
+
+<br>
+*Figure 4: Linux boot status for ruby memory system and systemd boot*
+
+The raw data/results of these experiments is available from this [link](http://dist.gem5.org/boot-test-results/boot_tests.zip) (warning: file size of ~40MB).
+
+## Moving Forward
+
+Researchers mostly fast-forward simulations during Linux boot to avoid the problems seen above or end up using older kernel versions.
+This leads to uncertainty in simulation results and conclusions drawn from such experiments.
+As a community, we should not overlook these problems and try to enable gem5 to successfully run these boot tests.
+There are a few JIRA issues
+([GEM5-359](https://gem5.atlassian.net/projects/GEM5/issues/GEM5-359), [GEM5-360](https://gem5.atlassian.net/projects/GEM5/issues/GEM5-360))
+open to document these problems with the hope of eventually fixing them.
+A gem5 [issue](https://gem5.googlesource.com/public/gem5/+/de24aafc161f348f678e0e0fc30b1ff2d145043b) related to TimingSimple CPU with Ruby memory system has already been fixed on the develop branch and will be part of gem5-20.
+
+Furthermore, we need to repeat these tests for new releases of gem5, and as new Linux kernels become available.
+We hope to keep the gem5 community up-to-date with the outcome of these tests via a new page on the gem5 website soon.
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