Disk image basics
A disk device in gem5 gets its initial contents from a file called a disk image. This file stores all the bytes present on the disk just as you would find them on an actual device. Some other systems also use disk images which are in more complicated formats and which provide compression, encryption, etc. gem5 currently only supports raw images, so if you have an image in one of those other formats, you'll have to convert it into a raw image before you can use it in a simulation. There are often tools available which can convert between the different formats.
Because a disk image represents all the bytes on the disk itself, it contains more than just a file system. For harddrives on most systems, the image starts with a partition table. Each of the partitions in the table (frequently only one) is also in the image. If you want to manipulate the entire disk you'll use the entire image, but if you want to work with just one partition and/or the file system on it, you'll need to specifically select that part of the image. The losetup command (discussed below) has a -o option which lets you specify where to start in an image.
Linux supports loopback devices which are devices backed by files. By attaching one of these to your disk image, you can use standard Linux commands on it which normally run on real disk devices. You can use the mount command with the "loop" option to set up a loopback device and mount it somewhere. Unfortunately you can't specify an offset into the image, so that would only be useful for a file system image, not a disk image which is what you need. You can, however, use the lower level losetup command to set up a loopback device yourself and supply the proper offset. Once you've done that, you can use the mount command on it like you would on a disk partition, format it, etc. If you don't supply an offset the loopback device will refer to the whole image, and you can use your favorite program to set up the partitions on it.
Working with image files
Creating an empty image
To create an empty image from scratch, you'll need to create the file itself, partition it, and format (one of) the partition(s) with a file system.
Create the actual file
First, decide how large you want your image to be. It's a good idea to make it large enough to hold everything you know you'll need on it, plus some breathing room. If you find out later it's too small, you'll have to create a new larger image and move everything over. If you make it too big, you'll take up actual disk space unnecessarily and make the image harder to work with. Once you've decided on a size you'll want to actually create the file. Basically, all you need to do is create a file of a certain size that's full of zeros. One approach is to use the dd command to copy the right number of bytes from /dev/zero into the new file. Alternatively you could create the file, seek in it to the last byte, and write one zero byte. All of the space you skipped over will become part of the file and is defined to read as zeroes, but because you didn't explicitly write any data there, most file systems are smart enough to not actually store that to disk. You can create a large image that way but take up very little space on your physical disk. Once you start writing to the file later that will change, and also if you're not careful, copying the file may expand it to its full size.
First, find an available loopback device using the losetup command with the -f option.
Next, use losetup to attach that device to your image. If the available device was /dev/loop0 and your image is foo.img, you would use a command like this.
losetup /dev/loop0 foo.img
/dev/loop0 (or whatever other device you're using) will now refer to your entire image file. Use whatever partitioning program you like on it to set up one (or more) paritions. For simplicity it's probably a good idea to create only one parition that takes up the entire image. We say it takes up the entire image, but really it takes up all the space except for the partition table itself at the beginning of the file, and possibly some wasted space after that for DOS/bootloader compatibility.
From now on we'll want to work with the new partition we created and not the whole disk, so we'll free up the loopback device using losetup's -d option
losetup -d /dev/loop0
First, find an available loopback device like we did in the partitioning step above using losetup's -f option.
We'll attach our image to that device again, but this time we only want to refer to the partition we're going to put a file system on. For PC and Alpha systems, that partition will typically be one track in, where one track is 63 sectors and each sector is 512 bytes, or 63 * 512 = 32256 bytes. The correct value for you may be different, depending on the geometry and layout of your image. In any case, you should set up the loopback device with the -o option so that it represents the partition you're interested in.
losetup -o 32256 /dev/loop0 foo.img
Next, use an appropriate formating command, often mke2fs, to put a file system on the partition.
You've now successfully created an empty image file. You can leave the loopback device attached to it if you intend to keep working with it (likely since it's still empty) or clean it up using losetup -d.
losetup -d /dev/loop0
Mounting an image
To mount a file system on your image file, first find a loopback device and attach it to your image with an appropriate offset as described in the "Formatting" section above.
losetup -f losetup -o 32256 /dev/loop0 foo.img
Now simply use the "mount" command to mount the loopback device somewhere.
mount /dev/loop0 ~/gem5_image
To unmount an image, use the umount command like you normally would. Don't forget to clean up the loopback device attached to your image with the losetup -d command.
losetup -d /dev/loop0
It's a good idea to understand how to build an image in case something goes wrong or you need to do something in an unusual way. You can, however, use the gem5img.py script which will go through the process described above for you. You can use the "init" command to create an empty image, "new", "partition", or "format" to perform those parts of init independently, and "mount" or "umount" to mount or unmount an existing image.
Now that you can create an image file and mount it's file system, you'll want to actually put some files in it. You're free to use whatever files you want, but the gem5 developers have found that Gentoo stage3 tarballs are a great starting point. They're essentially an almost bootable and fairly minimal Linux installation and are available for a number of architectures.
If you choose to use a Gentoo tarball, first extract it into your mounted image. The /etc/fstab file will have placeholder entries for the root, boot, and swap devices. You'll want to update this file as apporpriate, deleting any entries you aren't going to use (the boot partition, for instance). Next, you'll want to modify the inittab file so that it uses the m5 utility program (described elsewhere) to read in the init script provided by the host machine and to run that. If you allow the normal init scripts to run, the workload you're interested in may take much longer to get started, you'll have no way to inject your own init script to dynamically control what benchmarks are started, for instance, and you'll have to interact with the simulation through a simulated terminal which introduces non-determinism.
By default gem5 does not store modifications to the disk back to the underlying image file. Any changes you make will be stored in an intermediate COW layer and thrown away at the end of the simulation. You can turn off the COW layer if you want to modify the underlying disk.
Kernel and bootloader
Also, generally speaking, gem5 skips over the bootloader portion of boot and loads the kernel into simulated memory itself. This means that there's no need to install a bootloader like grub to your disk image, and that you don't have to put the kernel you're going to boot from on the image either. The kernel is provided separately and can be changed out easily without having to modify the disk image.