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A Review of the qemu Virtual Machine on Debian Linux


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qemu -- What Is It?

Qemu is an open-source Virtual Machine. If you want to know more about the general concept of virtual machines, check out Virtual Machine Intro.

Qemu is one of the most easily available Virtual Machines in that it is open source and available for the most popular operating systems. Interface-wise, qemu is a minimalist product, using command line parameters to create and run guest operating systems.

The computer on which qemu is installed is called the host. Within the qemu engine can be installed other operating systems which run on the host. The operating systems installed in qemu are called guests.

Qemu can exist on Linux, Windows, and MacOSX hosts. On these respective hosts can be installed guest operating systems of Linux, Windows, and DOS.

To increase execution speed, kqemu accelerator packages are available for some architectures that let code run directly on the host hardware. Particularly, newer versions of Windows running in Linux can make use of these accelerators.

The result is that you can choose a host that you like for most of your work, and still be able to run other guest operating systems in a window on your host.

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How do you get Qemu?

In many versions of Linux, qemu is already included in the distribution. I use Debian Etch, and the entire qemu package (version 0.8.2) was ready to be installed by the Debian package manager.

The procedure is:

sudo apt-get install qemu qemu-launcher

This command installs both the qemu Virtual Machine and a GUI utility named qemu-launcher. Qemu-launcher makes it easier to configure and install new guests in the qemu Virtual Machine. Basically the qemu-launcher creates the qemu command line arguments from menu selections presented to the user.

Also included in the Debian distribution is the kqemu accelerator package. Kqemu, depending upon architecture and the guest system, can considerably speed up the performance of a guest OS.

To install the kqemu package, you'll first need to be sure that the header files for you currently running distribution are installed. If using Debian, do the following:

apt-get install linux-headers-`uname -r`

In Debian, the kqemu package needs to be installed using the module assistant utility. The procedure is:

sudo apt-get install kqemu-common kqemu-source
sudo module-assistant prepare kqemu
sudo module-assistant auto-install kqemu

Once installed, you can make use of the kqemu accelerator by either loading the module automatically at boot by listing the kqemu module in /etc/modules, or by doing a sudo modprobe kqemu prior to running qemu.

Frankly, I suggest you go directly to to get the latest version of qemu. Qemu is a relatively new product, and many additional features and fixes are being implemented in each revision.

I realize that getting a new version of something for Linux can lead to a difficult install sometimes. But installing the new qemu is very easy.

Just download the latest Linux qemu binary as a tar-zip, copy the file to the root directory, and as root, untar it. It will then automatically unpack into the proper /usr directories.

Installing the latest kqemu is also easy, though it does require you to have your Linux system headers installed. Using the make utility (check the kqemu README) provides an easy install.

If you decide to get the latest version, be sure to check the install instructions in case they've changed.

How Do You Install A Guest Into qemu?

To install a guest OS into qemu you can use qemu-launcher if you have it. It makes installing a new guest a simple point and click affair.

If you don't have qemu-launcher, you can put the guest OS into the cdrom and do the steps in the following example:

qemu-img qemu_winxp 5000M
qemu -m 256 -boot d -cdrom /dev/cdrom -hda qemu_winxp \
-net nic,vlan=0 -net user,vlan=0 -localtime -no-qemu

In the example, the first command uses the qemu-img command to create an image file for the new guest, called qemu_winxp in the example. The command also needs the file size, given as 5000M in the example, which is 5000 megabytes, or 5 gigabytes.

The second line launches the qemu Virtual Machine on the new file, instructing it to use no more than 256 megabytes of memory, boot from the cd (-boot d), and use /dev/cdrom for the cdrom. The remaining arguments setup the network capability and turn off the accelerator, a safe thing to do on install.

Note: If you are going to install Windows 2000, add the -win2k-hack parameter to prevent an erroneous disk-full error that will abort your install.

Incidentally, while qemu runs systems reasonably well, especially with the kqemu accelerator, it installs some guest systems very, very slowly. We may be talking a couple of hours. Why it is so slow I do not know.

How do you launch a qemu guest?

To launch qemu with the installed guest OS (Windows XP in this example):

qemu -m 256 -hda qemu_winxp -net nic,vlan=0 -net user,vlan=0 \
-localtime -kernel-kqemu

In this example, the kqemu accelerator is set to its highest level with the -kernel-kqemu parameter. Should qemu fail with this parameter, just remove the parameter which defaults to only using kqemu to accelerate user programs, and not the entire guest OS.

You can review the qemu man page or the qemu html documentation to see all the other parameters that can be included on the qemu command line. Additional parameters allow connections to parallel ports, serial ports, and usb, among other things.

When in a qemu guest window, you can switch to a qemu status window with the keys:


While in this window you can check the status of the kqemu accelerator with the info kqemu command, for example. If your command line turns on usb support with the -usb parameter you can use the status window to see the IDs of the plugged in devices and mount the one or ones you wish, or unmount them (if you've already unmounted them from the Windows guest). Just do a help command in the status window to see all the commands you may use there.

In my Debian system, I found that I had to make one system change in order to get usb working in qemu. Without the fix, trying to use usb in qemu failed because of a privilege issue. The fix involved getting group access to the usbfs virtual driver. This was accomplished as follows:

1) If you have the file /etc/init.d/, find the line:
domount usbfs usbdevs ...
2) Add to the line:
... ,devgid=1000,devmode=664
where in my case, 1000 was my group number.
3) Reboot, and now qemu should work with usb.

With this fix, the usb support of the 0.9.1 engine seems work well, though the mechanism is a bit clumsy compared to other Virtual Machines. You must mount a device by either its bus and id, or its product code and vendor id.

To exit the status window and get back to the guest OS window, use the following key combination:


To get keyboard and mouse control out of the guest window, use the CNTL-ALT key combination. To get back into the qemu guest window, just click the mouse in the guest window.

Networking with this qemu launch command is simple. Just set up your guest to use DHCP, and usually other guest network parameters can be left alone.

More advice on setting up qemu can be found at Qemu Installation Notes.

Personal Observations

I've tested qemu on Freedos, Windows NT, Windows 2000, and Windows XP guest systems. The Windows 2000 and Windows XP systems took a couple of hours to install. Freedos and Windows NT run quite well in my 1 gigahertz system. Windows 2000 and Windows XP are sluggish, I'm sure some of the sluggishness is because in my AMD architecture I can't successfully run the systems with full (-kernel-kqemu) acceleration.

I've also installed all of these operating systems into the VMware Server Virtual Machine. In all cases the installs took less time, and once the VMware additions were installed (in the Windows OS's) the systems all ran very well. No particular slowdown of any of the systems was noticed.

I also installed Windows 2000 into Virtualbox for comparison. Virtualbox runs guest systems as fast as VMware as near as I can tell, and much faster than qemu, at least when full use of kqemu can't be utilized.

Still, the handiness of launching an OS with a command file instead of a GUI is refreshing to me. So I use Freedos and Windows NT mostly from my qemu installs because of that ease of use.

One peculiarity I found was with the serial port implementation. I have a quickpad IR (reviewed on quickpad review). It comes with Windows software than can be used to move text files between the quickpad and the computer. The connection is through a serial port.

In the qemu guest I can verify that the serial port is working by using it to upload and download files to my HP48 calculator. Yet I cannot establish communication with the quickpad.

With Windows running in VMware, I can communicate with the quickpad. I don't yet know why the qemu serial port doesn't work with the quickpad interface.