Hello,

I am new to GEM5 and I would appreciate any guidance.
I would like to measure the performance of a assembly benchmark code. The benchmark accesses memory addresses where data is pre-dumped at these addresses in a RAW disk image. My current setup consist of the image disk (containing the data) generated using qemu-img and the compiled binary code.

I would appreciate if I can get answers to some or all my inquiries below as it can help me move forward:
1- When simulating in FS mode, is a kernel (e.g., linux kernel) a must or I can run a binary code without a kernel?
2- Does I/O devices (including a disk image) is mounted by the kernel or can be done systematically in the setup of GEM5?

Thanks,
Alain

Hello Jason,

Thank you for your response.
My benchmark tests memory access where I want to have a hard drive from which data is accessed (e.g., accessing data from a SATA hard drive).

I have two more questions:
1- from the documentation of GEM5, I learned that in FS mode the kernel and disk image containing the OS is passed to the system. Thus, is a minimal OS required as well or optional?
2- does FS system accepts a binary code as an input that executes after booting the kernel or the only procedure consist of: i) boot the kernel/OS; ii) mount disk (through kernel/minimal OS); and iii) execute the binary code from a mounted disk image?

Thanks,
Alain

On Feb 21, 2024, at 2:40 PM, Jason Lowe-Power jason@lowepower.com wrote:


Hi Alain,

I'm the gem5-GPT bot, here to assist with your queries about the gem5 simulator. Please note that while I aim to provide accurate information, my responses could be wrong or may have been slightly edited for clarity. You can access the gem5-GPT on chat.openai.com/g/g-o3y1y4t8J-gem5-gpthttp://chat.openai.com/g/g-o3y1y4t8J-gem5-gpt if you subscribe to chatGPT.

Welcome to the world of gem5, Alain! Let's address your inquiries one by one:

1. Full System (FS) Mode and the Requirement of a Kernel:
   In gem5's Full System (FS) mode, the simulation environment is designed to emulate an entire computer system, including processors, memory, and I/O devices. This mode indeed requires a kernel, as it simulates the hardware and software interactions that occur in a real system. The kernel is responsible for managing the hardware resources, including CPU scheduling, memory management, and the handling of I/O operations. Therefore, to run binary code in FS mode, you must have an operating system kernel (such as Linux) loaded and configured to bootstrap the system and execute your binary code.

2. Mounting I/O Devices, Including Disk Images:
   In FS mode, the mounting and management of I/O devices, including disk images, are typically handled by the kernel. The kernel uses device drivers to interact with the hardware simulated by gem5, such as disk drives. You can specify disk images in your gem5 simulation setup, and these will be recognized as physical disks by the simulated kernel, which can then mount the filesystems contained within them, similar to how a real system operates.

   However, you do have the ability to configure certain aspects of how these devices are presented to the simulated system through gem5's configuration scripts. For example, you can specify the disk image to use, the type of disk (e.g., IDE, SATA), and the connection point (e.g., primary master) in the system's configuration file. This setup is part of the simulation environment configuration and is done before starting the simulation.

To summarize, running assembly benchmark code that accesses memory addresses pre-dumped on a RAW disk image in FS mode requires:

• An operating system kernel to manage hardware interactions, including those with the disk image.
• Configuration of the gem5 simulation environment to include the disk image as part of the simulated system's hardware, which the kernel will then manage.

If you're looking to run binaries without involving a full operating system kernel, gem5's System Call Emulation (SE) mode is what you're looking for. SE mode allows you to run user-level applications directly on gem5, with the simulator providing a minimal environment to handle system calls made by the application. Here's a detailed overview of SE mode and its implications for your project:

SE Mode Overview

• Direct Execution of Binaries: SE mode enables the direct execution of compiled binary applications without the need for a full operating system. This mode is particularly useful for running benchmarks or specific application tests where the overhead and complexity of simulating an entire OS are unnecessary.

• System Call Emulation: gem5 in SE mode emulates the system calls that the application makes. This emulation is handled internally by gem5, which intercepts system calls and provides the necessary responses, simulating the effect of having an OS handle these calls. This includes calls for file I/O, memory allocation, and process control, among others.

• Simplified I/O and Filesystem Interaction: Although SE mode can emulate system calls related to file I/O, the way it handles disk images and I/O devices is different from FS mode. You can specify input and output files for your application through gem5's command-line options, but the direct simulation of disk images as block devices (like in FS mode) is not supported. Instead, you would typically use virtual filesystems or specify file paths that gem5 will redirect to real files on your host system.

Running Binaries in SE Mode

To run a binary in SE mode, you'll generally follow these steps:

1. Compile Your Binary: Ensure your benchmark or application is compiled for the target architecture you intend to simulate (e.g., ARM, x86).

2. Set Up Your gem5 Simulation Script: Configure a gem5 simulation script to run in SE mode. This script specifies the binary to execute, along with any arguments or environment variables needed by the application.

3. Specify File Inputs/Outputs: If your application expects to read from or write to files, you can use gem5's options to map these files within the simulation environment. This allows your application to interact with files as if it were running on a real system.

4. Run the Simulation: Execute your simulation script with gem5, which will start the application in SE mode, handle its system calls, and simulate its execution on the specified architecture.

Limitations and Considerations

• Limited Device Interaction: SE mode doesn't simulate hardware devices or a full network stack as comprehensively as FS mode. If your application requires complex interactions with hardware, FS mode might be more appropriate.

• Memory Access and Management: While SE mode can simulate memory accesses made by your application, the nuances of how memory is managed might differ from a real system with a full kernel.

SE mode can be a powerful tool for focused performance analysis and testing of specific applications or benchmarks without the overhead of a full operating system. However, understanding its limitations and the scope of its system call emulation is crucial for setting realistic expectations for your simulation results.

On Wed, Feb 21, 2024 at 8:06 AM Alain Aoun via gem5-users <gem5-users@gem5.orgmailto:gem5-users@gem5.org> wrote:
Hello,

I am new to GEM5 and I would appreciate any guidance.
I would like to measure the performance of a assembly benchmark code. The benchmark accesses memory addresses where data is pre-dumped at these addresses in a RAW disk image. My current setup consist of the image disk (containing the data) generated using qemu-img and the compiled binary code.

I would appreciate if I can get answers to some or all my inquiries below as it can help me move forward:
1- When simulating in FS mode, is a kernel (e.g., linux kernel) a must or I can run a binary code without a kernel?
2- Does I/O devices (including a disk image) is mounted by the kernel or can be done systematically in the setup of GEM5?

Thanks,
Alain

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