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Arch Linux in QEMU for Raspberry Pi 2
#!/usr/bin/env bash
echo "Arch Linux in QEMU with love from eirikb"
set -x
wget "$TARGET"
qemu-img create arch.img 32G
fdisk arch.img <<EOF
LOOPS=$(sudo kpartx -av arch.img | grep -Po 'loop.p.')
BOOT=$(echo "$LOOPS" | head -n1)
ROOT=$(echo "$LOOPS" | tail -n1)
sudo mkfs.vfat /dev/mapper/"$BOOT"
sudo mkfs.ext4 /dev/mapper/"$ROOT"
sudo rm -rf boot root b
mkdir boot root b
sudo mount /dev/mapper/"$BOOT" boot
sudo mount /dev/mapper/"$ROOT" root
sudo bsdtar -xpf "$TARGET" -C root
sudo sync
sudo mv root/boot/* boot
sudo cp -r boot/* b/
sudo umount boot root
sudo kpartx -d arch.img
sudo rm -rf root boot
echo 'qemu-system-arm \
-M raspi2b \
-kernel b/kernel7.img \
-drive "format=raw,file=arch.img" \
-append "console=ttyAMA0 root=/dev/mmcblk0p2 rw rootwait" \
-dtb b/bcm2709-rpi-2-b.dtb \
-serial stdio \
-display none' >
# How I think it works, people please correct me if I'm wrong, I don't know much about this:
# -M raspi2:
# QEMU has a 'machine' raspi2, this will set up correct hardware on correct locations/buses (network, usb etc.)
# -kernel b/kernel7.img:
# Even though kernel is located on the disk QEMU must load it from outside the guest (from host).
# Raspberry has a two-stage bootloader, where the first bootloader is on-board (hardware).
# QEMU doesn't have this, and therefore can't simply kick start the second stage (called bootcode.bin).
# Instead kernel is loaded and run from outside, and no bootloader is used at all.
# -append "console=ttyAMA0 root=/dev/mmcblk0p2 rootwait":
# Append flags for kernel, these will be passed to the preivously loaded kernel.
# console=ttyAMA0 is used for virtual console, this makes QEMU able to print and read to/from stdio.
# root is where the kernel can find the root disk. I'm not sure about the path, it seems wrong to me.
# For the first I don't usee -sd (see below) so I expected /dev/sda, not /dev/mmcblk0p2.
# Secondly I expected this path to be hard coded as default in the kernel (that's the sd card path on rpi).
# rootwait does this: . Basically wait for rootfs to be available.
# Without number provided it probably do polling. Not 100% why this is required, beside the obivous that rootfs isn't availalbe at correct time.
# -dtb b/bcm2709-rpi-2-b.dtb:
# Set device tree binary. This is a file describing where devices are located for the kernel. Devices on rpi are non-discoverable, and the same
# kernel supports multiple (all) versions of raspberry. I'm not sure if QEMU uses this internally, beside passing it to kernel, in theory it should be enough with -M raspi2.
# An alternative is to have a kernel with this built into the kernel. I believe some OSes do this. archisoarm, at least for rpi, have these available for multiple rpi versions.
# Tip: You can see them by "ls b" after running this script.
# -drive "format=raw,file=arch.img":
# Tell QEMU about where the disk is. This disk will be created by this script. Used -drive instead of -sd in order to remove the warning about "raw" format.
# -serial stdio -display none:
# Make QEMU use terminal for input/output. Note ctrl+c closes. Change this as needed
set +x
echo 'All is well. To run: sh Give it some seconds'
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The drive location parameter is very flaky in x86 as well. The drive you provided (drive=whatever) could become visible as sda or sdb or anything else after each run. I'm not sure why, but I think the reason in x86 was that QEMU was creating an extra drives as sda or sdb for some propose thus the drive provided by me became sdb, and thus kernel panicked because sda didn't contain any rootfs.

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eirikb commented Jul 13, 2021

@ubdussamad I expected /dev/sda, was surprised it worked. I wonder if it could be something -m raspi2 does. It would be safer to use -sd, but then I got a warning about raw image, and I didn't find an option to set it. Perhaps if the image is converted to qcowit will detect the type automatically and not complain.

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