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+.. SPDX-License-Identifier: GPL-2.0+
+.. sectionauthor:: Bin Meng <bmeng.cn@gmail.com>
+
+QEMU x86
+========
+
+Build instructions for bare mode
+--------------------------------
+
+To build u-boot.rom for QEMU x86 targets, just simply run::
+
+   $ make qemu-x86_defconfig (for 32-bit)
+   $ make qemu-x86_64_defconfig (for 64-bit)
+   $ make all
+
+Note this default configuration will build a U-Boot for the QEMU x86 i440FX
+board. To build a U-Boot against QEMU x86 Q35 board, you can change the build
+configuration during the 'make menuconfig' process like below::
+
+   Device Tree Control  --->
+       ...
+       (qemu-x86_q35) Default Device Tree for DT control
+
+Test with QEMU for bare mode
+----------------------------
+
+QEMU is a fancy emulator that can enable us to test U-Boot without access to
+a real x86 board. Please make sure your QEMU version is 2.3.0 or above test
+U-Boot. To launch QEMU with u-boot.rom, call QEMU as follows::
+
+   $ qemu-system-i386 -nographic -bios path/to/u-boot.rom
+
+This will instantiate an emulated x86 board with i440FX and PIIX chipset. QEMU
+also supports emulating an x86 board with Q35 and ICH9 based chipset, which is
+also supported by U-Boot. To instantiate such a machine, call QEMU with::
+
+   $ qemu-system-i386 -nographic -bios path/to/u-boot.rom -M q35
+
+Note by default QEMU instantiated boards only have 128 MiB system memory. But
+it is enough to have U-Boot boot and function correctly. You can increase the
+system memory by pass '-m' parameter to QEMU if you want more memory::
+
+   $ qemu-system-i386 -nographic -bios path/to/u-boot.rom -m 1024
+
+This creates a board with 1 GiB system memory. Currently U-Boot for QEMU only
+supports 3 GiB maximum system memory and reserves the last 1 GiB address space
+for PCI device memory-mapped I/O and other stuff, so the maximum value of '-m'
+would be 3072.
+
+QEMU emulates a graphic card which U-Boot supports. Removing '-nographic' will
+show QEMU's VGA console window. Note this will disable QEMU's serial output.
+If you want to check both consoles, use '-serial stdio'.
+
+Multicore is also supported by QEMU via '-smp n' where n is the number of cores
+to instantiate. Note, the maximum supported CPU number in QEMU is 255.
+
+U-Boot uses 'distro_bootcmd' by default when booting on x86 QEMU. This tries to
+load a boot script, kernel, and ramdisk from several different interfaces. For
+the default boot order, see 'qemu-x86.h'. For more information, see
+'README.distro'. Most Linux distros can be booted by writing a uboot script.
+For example, Debian (stretch) can be booted by creating a script file named
+'boot.txt' with the contents::
+
+   setenv bootargs root=/dev/sda1 ro
+   load ${devtype} ${devnum}:${distro_bootpart} ${kernel_addr_r} /vmlinuz
+   load ${devtype} ${devnum}:${distro_bootpart} ${ramdisk_addr_r} /initrd.img
+   zboot ${kernel_addr_r} - ${ramdisk_addr_r} ${filesize}
+
+Then compile and install it with::
+
+   $ apt install u-boot-tools && \
+     mkimage -T script -C none -n "Boot script" -d boot.txt /boot/boot.scr
+
+The fw_cfg interface in QEMU also provides information about kernel data,
+initrd, command-line arguments and more. U-Boot supports directly accessing
+these informtion from fw_cfg interface, which saves the time of loading them
+from hard disk or network again, through emulated devices. To use it , simply
+providing them in QEMU command line::
+
+   $ qemu-system-i386 -nographic -bios path/to/u-boot.rom -m 1024 \
+     -kernel /path/to/bzImage -append 'root=/dev/ram console=ttyS0' \
+     -initrd /path/to/initrd -smp 8
+
+Note: -initrd and -smp are both optional
+
+Then start QEMU, in U-Boot command line use the following U-Boot command to
+setup kernel::
+
+   => qfw
+   qfw - QEMU firmware interface
+
+   Usage:
+   qfw <command>
+       - list                             : print firmware(s) currently loaded
+       - cpus                             : print online cpu number
+       - load <kernel addr> <initrd addr> : load kernel and initrd (if any) and setup for zboot
+
+   => qfw load
+   loading kernel to address 01000000 size 5d9d30 initrd 04000000 size 1b1ab50
+
+Here the kernel (bzImage) is loaded to 01000000 and initrd is to 04000000. Then,
+'zboot' can be used to boot the kernel::
+
+   => zboot 01000000 - 04000000 1b1ab50
+
+To run 64-bit U-Boot, qemu-system-x86_64 should be used instead, e.g.::
+
+   $ qemu-system-x86_64 -nographic -bios path/to/u-boot.rom
+
+A specific CPU can be specified via the '-cpu' parameter but please make
+sure the specified CPU supports 64-bit like '-cpu core2duo'. Conversely
+'-cpu pentium' won't work for obvious reasons that the processor only
+supports 32-bit.
+
+Note 64-bit support is very preliminary at this point. Lots of features
+are missing in the 64-bit world. One notable feature is the VGA console
+support which is currently missing, so that you must specify '-nographic'
+to get 64-bit U-Boot up and running.