QEMU with U-Boot uses 100% of CPU - virtualization

Question

Inside a Lubuntu VirtualBox VM (macOS host) I'm running QEMU with a U-Boot kernel (?) to load my compiled ARM assembly code (using tftp addr *.bin and go addr).
My problem is that QEMU uses 100% of the CPU. I'm following an Assembly programming course and was told that it wasn't normal (that's also why I'm using a VM)…
I'm not sure where does the problem come from, is it a config issue with VirtualBox (Guest additions are installed) or with QEMU?
QEMU is launched with the following arguments:
export QEMU_AUDIO_DRV=none
qemu-arm/arm-softmmu/qemu-system-arm -s -localtime -m 256 -M reptar -kernel u-boot-arm/u-boot -tftp . -serial mon:stdio -nographic
Version is:
$ qemu-arm/arm-softmmu/qemu-system-arm --version
QEMU emulator version 2.4.1 (qemu-linaro from git), Copyright (c) 2003-2008 Fabrice Bellard
Boot output:
reptar-sp6-emul: sp6_emul_init
sp6_emul_init: failed to connect to SP6 server
sp6_emul_init: terminate thread
U-Boot 2016.09 (Sep 26 2018 - 17:19:23 +0200)
OMAP35XX-GP ES3.1, CPU-OPP2, L3-165MHz, Max CPU Clock 600 MHz
REDS Reptar board + LPDDR/NAND
I2C: ready
DRAM: 256 MiB
NAND: 256 MiB
MMC: OMAP SD/MMC: 0
Using default environment
Net: smc911x-0
Reptar #

Answer 1

QEMU will use host CPU when the guest CPU is doing something, even if that "something" is "spin around in a tight loop". If the guest code uses the CPU instruction WFI ("wait for interrupt") in its idle loop, QEMU will handle that by stopping until a guest interrupt occurs. (WFI is an Arm instruction; for other guest architectures there is generally an equivalent instruction, like x86's HLT.)
So whether QEMU uses 100% CPU when the guest is "idle" depends on what exactly the guest's idle loop does. The idle loop in a "real OS" like Linux will use WFI (which on real hardware is useful for reducing power consumption) and so will idle at very low host CPU usage. CPU usage sitting at a u-boot prompt will depend on how u-boot is coded; CPU usage in your assembly code will depend on what it does.

Answer 2

You can use cpulimit with -l flag, to limit processor time for particular process. For example:
cpulimit -l 20 -- qemu-system-x86_64 -bios ./u-boot.rom
will limit qemu cpu usage by 20% of one core.
Also, more tricky way on native hardware (probably will not work in virtualbox): if you've got multicore processor, you may dedicate 1 cpu core for qemu, and limit it's clock frequency to lowest possible value, to save energy and keep it cool. Example on my Ryzen 4650u cpu:
install cpupower and tasksel: apt install linux-cpupower tasksel
get lowest possible core clock: cpupower -c 0 frequency-info -l
set core 0 clock frequency to lowest possible value: cpupower -c 0 frequency-set -u clock_freq 1400Mhz
run u-boot in qemu, assign process to core 0: qemu-system-x86_64 -bios ./u-boot.rom & taskset -cp 0 $!