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.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Closes #16924

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])
 ])

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=''

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:

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

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)