Archipelago » qemu

performance. On an x86 host, if you want the highest performance for

the x86 target, the @emph{QEMU Accelerator Module} (KQEMU) allows QEMU

to reach near native performances. KQEMU is currently only supported

for an x86 Linux 2.4 or 2.6 host system, but more host OSes will be

supported in the future.

@item -no-kqemu

Disable the usage of the QEMU Accelerator module (KQEMU). QEMU will work as

usual but will be slower. This option can be useful to determine if

emulation problems are coming from KQEMU.

@section QEMU Accelerator Module

The QEMU Accelerator Module (KQEMU) is an optional part of QEMU currently only

available for Linux 2.4 or 2.6 x86 hosts. It enables QEMU to run x86

code much faster. Provided it is installed on your PC (see

@ref{kqemu_install}), QEMU will automatically use it.

WARNING: as with any alpha stage kernel driver, KQEMU may cause

arbitrary data loss on your PC, so you'd better backup your sensitive

data before using it.

When using KQEMU, QEMU will create a big hidden file containing the

RAM of the virtual machine. For best performance, it is important that

this file is kept in RAM and not on the hard disk. QEMU uses the

@file{/dev/shm} directory to create this file because @code{tmpfs} is

usually mounted on it (check with the shell command

@code{df}). Otherwise @file{/tmp} is used as fallback. You can use the

@var{QEMU_TMPDIR} shell variable to set a new directory for the QEMU

RAM file.

KQEMU has only been tested with Linux 2.4, Linux 2.6 and Windows 2000

as guest OSes. If your guest OS do not work with KQEMU, you can

dynamically disable KQEMU with the @option{-no-kqemu} option.

If you want to use the optional QEMU Accelerator Module, then download

it and install it in qemu-x.y.z/:

@example

tar zxvf /tmp/kqemu-x.y.z.tar.gz

@end example

@node kqemu_install

@subsection QEMU Accelerator Installation

If you use x86 Linux, the compilation of the QEMU Accelerator Kernel

Module (KQEMU) is automatically activated provided you have the

necessary kernel headers. If nonetheless the compilation fails, you

can disable its compilation with the @option{--disable-kqemu} option. 

If you are using a 2.6 host kernel, then all the necessary kernel

headers should be already installed. If you are using a 2.4 kernel,

then you should verify that properly configured kernel sources are

installed and compiled. On a Redhat 9 distribution for example, the

following must be done:

@example

1) Install the kernel-source-xxx package

2) cd /usr/src/linux-xxx

3) make distclean

4) Copy /boot/config-vvv in .config (use uname -r to know your configuration name 'vvv')

5) Edit the Makefile to change the EXTRAVERSION line to match your

   current configuration name:

   EXTRAVERSION = -custom

to 

   EXTRAVERSION = -8 # This is an example, it can be -8smp too

5) make menuconfig # Just save the configuration

6) make dep bzImage

@end example

The installation of KQEMU is not fully automatic because it is highly

distribution dependent. When launching 

@example

make install

@end example

KQEMU is installed in /lib/modules/@var{kernel_version}/misc. The

device @file{/dev/kqemu} is created with read/write access rights for

everyone. If you fear security issues, you can restrict the access

rights of @file{/dev/kqemu}.

If you want that KQEMU is installed automatically at boot time, you can add

@example

# Load the KQEMU kernel module

/sbin/modprobe kqemu

@end example

in @file{/etc/rc.d/rc.local}.

If your distribution uses udev (like Fedora), the @file{/dev/kqemu} is

not created automatically (yet) at every reboot. You can add the

following in @file{/etc/rc.d/rc.local}:

@example

# Create the KQEMU device

mknod /dev/kqemu c 250 0

chmod 666 /dev/kqemu

@end example