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QEMU: Difference between revisions
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{{Emulators}}
QEMU is a free and open-source emulator that performs hardware virtualization. It is widely available for variety of host operating-systems and requires minimal configuration for use in operating-system development. It is capable of emulating a wide variety of systems including [[ARM]], [[x86]] and [[RISC-V]], among others. For a more comprehensive list of targets refer to the [https://www.qemu.
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* PCI SVGA card (Cirrus Logic 5446)
* PCI support (With BIOS32).
* [[QEMU_fw_cfg]] Allows you to pass info to the guest
==Usage==
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QEMU does not require the use of a configuration script like Bochs. An example usage of QEMU can be seen below:
<
qemu-system-i386 \
-accel tcg,thread=single \
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-usb \
-vga std
</syntaxhighlight>
When running QEMU on a non-UNIX system, it may be necessary to use the <tt>-L</tt> command-line option to instruct QEMU where to find a BIOS image.
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The <tt>-m 128</tt> argument instructs QEMU to create the guest system with 128MB of RAM. The <tt>-drive format=raw,media=cdrom,file=myos.iso</tt> argument instructs QEMU to create a drive in our guest system. In this case the created drive is a CDROM drive, with <tt>myos.iso</tt> as its loaded media. If more than one drive in is required in the guest system, it is possible to use the <tt>-boot</tt> option to define the order in which the system will check for bootable media.
The <tt>-serial stdio</tt> argument used above instructs QEMU to redirect the serial input and output to the host system's <tt>stdio</tt> stream. This is particularly useful for debugging purposes. In the documentation, the option to redirect this to <tt>stdio</tt> is described as Unix only. Presumably, one should be able to achieve a similar effect with <tt>-serial file:CON</tt> on Windows.
In order to help track down the source of a triple fault, the <tt>-d int</tt> option can be used to show additional debugging information on interrupts that occur. Additionally, the <tt>-no-shutdown</tt> and <tt>-no-reboot</tt> options can be used. These instruct the virtual machine to halt on error instead of rebooting or shutting down, allowing for the operator to inspect the state of the machine after a triple-fault.
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Additional useful command-line options are listed within the [[QEMU#Useful_QEMU_command-line_options|Useful QEMU command-line options]] section.
The keyboard shortcuts <tt>CTRL-ALT-{1,2,3}</tt> can be used inside the emulator to swap in/out of the emulation screen, the QEMU
===The
Instead of the serial port, as outlined above, one can also use the <tt>debugcon</tt> device. By running QEMU with <tt>-debugcon stdio</tt>, you can print a message to your host system's terminal by just writing each byte to port 0xe9. This is the same port as the one used by [[Bochs#I/O_debugger_macros|the equivalent feature in Bochs]].
Unlike the serial port, the debug console is one-way – you can only output. On the flip side, implementing support is much easier, and outputting the messages isn't artificially throttled by a baudrate.
===The QEMU monitor===
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It is possible to redirect QEMU's monitor output to stdio using the following command-line option when invoking QEMU:
<
-monitor stdio
</syntaxhighlight>
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For convenience, it is also possible to create a file containing commands for GDB to automatically execute. GDB will read and execute the contents of a file with the name <tt>.gdbinit</tt> in the current working directory. Alternatively, a different file may be specified by the use the <tt>-command=file</tt> command-line argument. An example file can be seen below:
<
<syntaxhighlight lang="text">
file <my-kernel-binary>
target remote localhost:1234
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# Inspect page tables
x /8wg &page_tables_start
</syntaxhighlight>
This will automatically load the kernel binary's symbol file into the debugger and then open the remote connection to QEMU.<br/>
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Some files within the QEMU source code have commented lines of the form:
<
// #define DEBUG_*
</syntaxhighlight>
Recompiling QEMU from source with this preprocessor variable defined will cause additional debugging information from the applicable file to be printed to stdout.
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