kpatch-build: fix find_parent_obj KVM shortcut for all arches

[jexbow]

Summary

The find_parent_obj() shortcut for virt/kvm/* files hardcodes arch/x86/kvm, which only works when the target arch is x86. On every other architecture the parent-object lookup for KVM files misses the shortcut and falls through to a deep (and slow) full-tree search — and as reported in #1519, on arm64 it can even miss the parent entirely.

Instead of adding per-arch if blocks, derive the arch-specific KVM directory from $TARGET_ARCH. The kpatch TARGET_ARCH values don't always match the kernel arch directory names, so a small mapping is needed:

kpatch TARGET_ARCH	kernel arch dir
x86_64	arch/x86/kvm
aarch64	arch/arm64/kvm
ppc64le	arch/powerpc/kvm
s390x	arch/s390/kvm
loongarch64	arch/loongarch/kvm

Verification

All kpatch-supported arches build the common KVM code from virt/kvm/ (v4.19: arm64/x86/s390/powerpc use KVM=../../../virt/kvm in their KVM Makefiles; modern kernels set KVM ?= ../../../virt/kvm in virt/kvm/Makefile.kvm; loongarch follows the same pattern). So the virt/kvm/* shortcut now correctly maps to the right arch directory on every supported arch.

Tested only on arm64 (4.19 kernel, gcc 8.2) — I don't have hardware for the other arches, but the change is mechanical and the mapping table above is complete.

Fixes: #1519

[joe-lawrence]

Thanks, @jexbow two really small nits:

• Can you add your "Signed-off-by: " line to the commit?
• The code looks like it would work, but how about:
  • Consolidating into the existing kernel_make_arch() -> rename to kernel_src_arch()
  • Add the arches
  • Then the fix becomes: find_parent_obj_in_dir "$file" "arch/$(kernel_make_arch "$TARGET_ARCH")/kvm"

[jexbow]

Thanks @joe-lawrence, both make sense.

1. Signed-off-by — added.

2. Consolidate into kernel_make_arch() — good call. The current kernel_make_arch() only maps aarch64→arm64 and passes the rest through unchanged, which is also a latent bug for x86_64/ppc64le/s390x/loongarch64 (kernel make ARCH= expects the source arch name). I renamed it to kernel_src_arch(), extended it to handle all kpatch-supported arches, and updated both call sites (the KARCH= one and the new find_parent_obj shortcut), which simplifies the fix to:

   find_parent_obj_in_dir "$file" "arch/$(kernel_src_arch "$TARGET_ARCH")/kvm"

Just force-pushed the update (amended commit) with the Signed-off-by line.

[jexbow]

Thanks @jpoimboe for the review! And thanks again @joe-lawrence for the suggestion to consolidate the arch mapping — that made the fix much cleaner.