From a29ff721c016a2db8fe57f4194ec856f823ec16a Mon Sep 17 00:00:00 2001
From: Phil Sutter <phil@nwl.cc>
Date: Fri, 3 Jan 2025 16:30:24 +0100
Subject: [PATCH] Support for cross-compiling kernel modules

In order to correctly cross-compile, one has to pass ARCH and
CROSS_COMPILE make flags to kernel module build calls. Facilitate this
in the same way as for custom CC flag by recognizing KERNEL_-prefixed
configure environment variables of same name.

Signed-off-by: Phil Sutter <phil@nwl.cc>
---
 config/kernel.m4             | 5 +++++
 config/zfs-build.m4          | 2 ++
 module/Makefile.in           | 2 ++
 rpm/generic/zfs-kmod.spec.in | 4 +++-
 rpm/redhat/zfs-kmod.spec.in  | 4 +++-
 5 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/config/kernel.m4 b/config/kernel.m4
index ae66633907bf..9928ead1b6ce 100644
--- a/config/kernel.m4
+++ b/config/kernel.m4
@@ -681,11 +681,16 @@ AC_DEFUN([ZFS_LINUX_COMPILE], [
 		building kernel modules])
 	AC_ARG_VAR([KERNEL_LLVM], [Binary option to
 		build kernel modules with LLVM/CLANG toolchain])
+	AC_ARG_VAR([KERNEL_CROSS_COMPILE], [Cross compile prefix
+		for kernel module builds])
+	AC_ARG_VAR([KERNEL_ARCH], [Architecture to build kernel modules for])
 	AC_TRY_COMMAND([
 	    KBUILD_MODPOST_NOFINAL="$5" KBUILD_MODPOST_WARN="$6"
 	    make modules -k -j$TEST_JOBS ${KERNEL_CC:+CC=$KERNEL_CC}
 	    ${KERNEL_LD:+LD=$KERNEL_LD} ${KERNEL_LLVM:+LLVM=$KERNEL_LLVM}
 	    CONFIG_MODULES=y CFLAGS_MODULE=-DCONFIG_MODULES
+	    ${KERNEL_CROSS_COMPILE:+CROSS_COMPILE=$KERNEL_CROSS_COMPILE}
+	    ${KERNEL_ARCH:+ARCH=$KERNEL_ARCH}
 	    -C $LINUX_OBJ $ARCH_UM M=$PWD/$1 >$1/build.log 2>&1])
 	AS_IF([AC_TRY_COMMAND([$2])], [$3], [$4])
 ])
diff --git a/config/zfs-build.m4 b/config/zfs-build.m4
index c44a893bbb8c..55fc029f0847 100644
--- a/config/zfs-build.m4
+++ b/config/zfs-build.m4
@@ -393,6 +393,8 @@ AC_DEFUN([ZFS_AC_RPM], [
 		RPM_DEFINE_KMOD=${RPM_DEFINE_KMOD}' --define "kernel_cc KERNEL_CC=$(KERNEL_CC)"'
 		RPM_DEFINE_KMOD=${RPM_DEFINE_KMOD}' --define "kernel_ld KERNEL_LD=$(KERNEL_LD)"'
 		RPM_DEFINE_KMOD=${RPM_DEFINE_KMOD}' --define "kernel_llvm KERNEL_LLVM=$(KERNEL_LLVM)"'
+		RPM_DEFINE_KMOD=${RPM_DEFINE_KMOD}' --define "kernel_cross_compile KERNEL_CROSS_COMPILE=$(KERNEL_CROSS_COMPILE)"'
+		RPM_DEFINE_KMOD=${RPM_DEFINE_KMOD}' --define "kernel_arch KERNEL_ARCH=$(KERNEL_ARCH)"'
 	])
 
 	RPM_DEFINE_DKMS=''
diff --git a/module/Makefile.in b/module/Makefile.in
index 9b34b3dfaec7..529ab81dcec5 100644
--- a/module/Makefile.in
+++ b/module/Makefile.in
@@ -55,6 +55,8 @@ modules-Linux:
 	mkdir -p $(sort $(dir $(zfs-objs) $(zfs-)))
 	$(MAKE) -C @LINUX_OBJ@ $(if @KERNEL_CC@,CC=@KERNEL_CC@) \
 		$(if @KERNEL_LD@,LD=@KERNEL_LD@) $(if @KERNEL_LLVM@,LLVM=@KERNEL_LLVM@) \
+		$(if @KERNEL_CROSS_COMPILE@,CROSS_COMPILE=@KERNEL_CROSS_COMPILE@) \
+		$(if @KERNEL_ARCH@,ARCH=@KERNEL_ARCH@) \
 		M="$$PWD" @KERNEL_MAKE@ CONFIG_ZFS=m modules
 
 modules-FreeBSD:
diff --git a/rpm/generic/zfs-kmod.spec.in b/rpm/generic/zfs-kmod.spec.in
index 30524474d1ac..7ed828bd0c9c 100644
--- a/rpm/generic/zfs-kmod.spec.in
+++ b/rpm/generic/zfs-kmod.spec.in
@@ -144,7 +144,9 @@ for kernel_version in %{?kernel_versions}; do
         %{debuginfo} \
         %{?kernel_cc} \
         %{?kernel_ld} \
-        %{?kernel_llvm}
+        %{?kernel_llvm} \
+        %{?kernel_cross_compile} \
+        %{?kernel_arch}
 
     # Pre-6.10 kernel builds didn't need to copy over the source files to the
     # build directory.  However we do need to do it though post-6.10 due to
diff --git a/rpm/redhat/zfs-kmod.spec.in b/rpm/redhat/zfs-kmod.spec.in
index 876c198c64de..a95bdf20f873 100644
--- a/rpm/redhat/zfs-kmod.spec.in
+++ b/rpm/redhat/zfs-kmod.spec.in
@@ -69,7 +69,9 @@ fi
         %{debuginfo} \
         %{?kernel_cc} \
         %{?kernel_ld} \
-        %{?kernel_llvm}
+        %{?kernel_llvm} \
+        %{?kernel_cross_compile} \
+        %{?kernel_arch}
 make %{?_smp_mflags}
 
 # Module signing (modsign)