Hi,

For testing drgn [1], we trigger a crash in a virtual machine (using
QEMU system emulation on an x86-64 host) and use /proc/vmcore. On Arm32,
/proc/vmcore is missing in the kdump environment, apparently due to
this:

  ------------[ cut here ]------------
  WARNING: CPU: 0 PID: 1 at arch/arm/mm/ioremap.c:293 __arm_ioremap_pfn_caller+0xb0/0x190
  Modules linked in:
  CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.12.0-rc1-vmtest32.1.1default #1
  Hardware name: Generic DT based system
  Call trace:
   unwind_backtrace from show_stack+0x10/0x14
   show_stack from dump_stack_lvl+0x68/0x74
   dump_stack_lvl from __warn+0x74/0x120
   __warn from warn_slowpath_fmt+0xa0/0x128
   warn_slowpath_fmt from __arm_ioremap_pfn_caller+0xb0/0x190
   __arm_ioremap_pfn_caller from __arm_ioremap_caller+0x40/0x48
   __arm_ioremap_caller from copy_oldmem_page+0x24/0x98
   copy_oldmem_page from read_from_oldmem+0xb0/0xf0
   read_from_oldmem from elfcorehdr_read.localalias+0x60/0x88
   elfcorehdr_read.localalias from vmcore_init+0x6c/0x414
   vmcore_init from do_one_initcall+0x88/0x1a0
   do_one_initcall from kernel_init_freeable+0x1ac/0x1f4
   kernel_init_freeable from kernel_init+0x14/0x12c
   kernel_init from ret_from_fork+0x14/0x28
  Exception stack(0x90839fb0 to 0x90839ff8)
  9fa0:                                     00000000 00000000 00000000 00000000
  9fc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
  9fe0: 00000000 00000000 00000000 00000000 00000013 00000000
  ---[ end trace 0000000000000000 ]---
  Kdump: vmcore not initialized

which is this warning:

	/*
	 * Don't allow RAM to be mapped with mismatched attributes - this
	 * causes problems with ARMv6+
	 */
	if (WARN_ON(memblock_is_map_memory(PFN_PHYS(pfn)) &&
		    mtype != MT_MEMORY_RW))
		return NULL;

Is this a known problem, or is something wrong with our configuration?
Our QEMU command line is:

  qemu-system-arm \
	-M virt,highmem=off \
	-smp 1 -m 2G \
  	-display none -serial mon:stdio -no-reboot \
	-virtfs local,id=root,path=build/vmtest/arm/rootfs,mount_tag=/dev/root,security_model=none,readonly=on,multidevs=remap \
	-virtfs local,path=/,mount_tag=host,security_model=none,readonly=on,multidevs=remap \
	-device virtio-rng \
	-device virtio-serial \
	-chardev socket,id=vmtest,path=/tmp/drgn-vmtest-ulmi3vhr/socket \
	-device virtserialport,chardev=vmtest,name=com.osandov.vmtest.0 \
	-drive file=/tmp/drgn-vmtest-ulmi3vhr/disk,if=virtio,media=disk,format=raw \
	-kernel build/vmtest/arm/kernel-6.12.0-rc1-vmtest32.1.1default/vmlinuz \
	-append 'rootfstype=9p rootflags=trans=virtio,cache=loose,msize=1048576 ro console=ttyAMA0,115200 panic=-1 crashkernel=256M'

I've attached the kernel configuration.

I found a 10-year-old thread about this [2], but there have apparently
been fixes to the Arm kdump code in the meantime, so it must have worked
at some point.

Thanks,
Omar

1: https://github.com/osandov/drgn
2: https://lore.kernel.org/linux-arm-kernel/132601cf5e89$94556a70$bd003f50$@gmail.com/T/#u