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13 July 2023

kdump on 32-bit Arm

by Petr Tesařík

NOTE: This post is about openSUSE Tumbleweed only.


AFAIK there are no ISO installation media, but there are plenty of pre-built images. The kdump package is currently not built, and there are good reasons for that. Read on.

The UEFI Debacle

Arm QEMU VMs use AAVMF firmware by default. However, the 32-bit AAVMF firmware cannot find any disks, rendering the system unbootable. This is a bug that should be fixed, but I haven’t had time to debug this any further.

I was able to boot the system with U-Boot, though. The U-Boot image for QEMU is not packaged, but you can build it from sources with make qemu_arm_defconfig. Install it to some convenient place; I installed mine as /usr/share/qemu/u-boot-qemu-arm.bin and modified my libvirt VM configuration thus:

  <type arch='armv7l' machine='virt-8.0'>hvm</type>
  <loader readonly='yes' type='rom'>/usr/share/qemu/u-boot-qemu-arm.bin</loader>
  <bootmenu enable='no'/>

EFI variables are lost at that point, making the system fall back to the default EFI executable on boot, which is missing. Mount your EFI partition and copy EFI/opensuse/grubarm.efi to EFI/BOOT/bootarm.efi.

The system then boots fine with U-Boot’s minimal UEFI implementation.

Fighting with kexec-tools

Next, I added crashkernel=128M to the kernel command line, rebooted and tried loading the panic kernel with a standard initrd:

kexec -l -p /boot/zImage --initrd=/boot/initrd --append="console=ttyAMA0"`

Good, this seems to work. So, let’s crash the kernel:

echo c > /proc/sysrq-trigger

The kernel boots and spits messages on the serial console. Originally, I thought that Arm kernels are not relocatable, because there is no CONFIG_RELOCATABLE, but Arm can achieve similar results with CONFIG_AUTO_ZRELADDR and CONFIG_ARM_PATCH_PHYS_VIRT.

Anyway, one of the kernel messages complains about invalid magic at start of compressed archive and eventually fails to mount root filesystem. That’s because the initramfs image is overwritten when the kernel uncompresses itself. This is also a bug that should be fixed, but meanwhile you can persuade kexec to load initrd higher in memory with e.g. --image-size=0x34c0000.

UEFI Strikes Again

The kernel can find the initramfs now, but it fails to initialize ELF core headers (elfcorehdr) with a fat kernel warning that the kernel is trying to remap normal RAM as an I/O region. This would indeed cause serious cache aliasing issues on Arm. However, ELF core headers are allocated at the top of crash kernel reserved region and removed from usable memory by kexec-tools, so this physical region should not be recognized as RAM by the panic kernel.

Hm. Turn on memblock debugging.

Ah. The panic kernel receives the unmodified EFI memory map and obediently processes all EFI_CONVENTIONAL_MEMORY descriptors. No, this processing cannot be turned off with noefi.

How to Make it Work

Since the panic kernel won’t work if the system was booted with UEFI, what about booting directly from U-Boot? Reboot the system and interrupt U-Boot to get the command line. Then boot Linux manually, e.g.:

fdt addr $fdtcontroladdr
fdt set /chosen bootargs "console=ttyAMA0 root=UUID=e370fb49-c0ea-4d6c-a788-727e11e87a81 rw crashkernel=128M"

My system has a separate /boot partition at vda2, so I load the kernel and initrd from there:

load virtio 0:2 $kernel_addr_r zImage-6.3.9-1-default
load virtio 0:2 $ramdisk_addr_r initrd-6.3.9-1-default

Note the size of the ramdisk; that’s the magic number below:

bootz $kernel_addr_r $ramdisk_addr_r:16959751 $fdtcontroladdr

When the system boots, load the panic kernel (see above) and crash the system. This time, you should get /proc/vmcore as expected.

You can also build and install kdump from the source repository. There are a few missing bits at the moment:

tags: kdump