[PATCH v2] tools/sched_ext: Improve cross-compilation support in Makefile

[yangsonghua]

Modify the tools/sched_ext/Makefile to better handle cross-compilation
environments by:

1. Adjusted `HOST_OUTPUT_DIR` to be relative to `$(OBJ_DIR)`, ensuring
   correct path handling during host tool building when cross-compile
   (HOST_OUTPUT_DIR now points to $(OBJ_DIR)/host-tools)
2. Properly propagate CROSS_COMPILE to libbpf sub-make invocation
3. Add missing $(HOST_BPFOBJ) build rule with proper host toolchain flags
   (ARCH=, CROSS_COMPILE=, explicit HOSTCC/HOSTLD)
4. Consistently quote $(HOSTCC) in bpftool build rule

The changes ensure proper cross-compilation behavior while maintaining
backward compatibility with native builds. Host tools are now correctly
built with the host toolchain while target binaries use the cross-toolchain.

Signed-off-by: yangsonghua <yangsonghua@xxxxxxxxxxx>

------
v2:
  - keep HOST_BUILD_DIR and LDFLAGS unchanged
  - change the title prefix "sched_ext" to "tools/sched_ext"

v1:
  - https://lore.kernel.org/lkml/20250414081436.2574882-1-yangsonghua@xxxxxxxxxxx/
---
 tools/sched_ext/Makefile | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/tools/sched_ext/Makefile b/tools/sched_ext/Makefile
index ca3815e572d8..eb85f3762a1f 100644
--- a/tools/sched_ext/Makefile
+++ b/tools/sched_ext/Makefile
@@ -62,7 +62,7 @@ BINDIR := $(OUTPUT_DIR)/bin
 BPFOBJ := $(BPFOBJ_DIR)/libbpf.a
 ifneq ($(CROSS_COMPILE),)
 HOST_BUILD_DIR		:= $(OBJ_DIR)/host
-HOST_OUTPUT_DIR	:= host-tools
+HOST_OUTPUT_DIR	:= $(OBJ_DIR)/host-tools
 HOST_INCLUDE_DIR	:= $(HOST_OUTPUT_DIR)/include
 else
 HOST_BUILD_DIR		:= $(OBJ_DIR)
@@ -136,14 +136,25 @@ $(MAKE_DIRS):
 $(BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile)			\
 	   $(APIDIR)/linux/bpf.h						\
 	   | $(OBJ_DIR)/libbpf
-	$(Q)$(MAKE) $(submake_extras) -C $(BPFDIR) OUTPUT=$(OBJ_DIR)/libbpf/	\
+	$(Q)$(MAKE) $(submake_extras) CROSS_COMPILE=$(CROSS_COMPILE) 		\
+		    -C $(BPFDIR) OUTPUT=$(OBJ_DIR)/libbpf/			\
 		    EXTRA_CFLAGS='-g -O0 -fPIC'					\
+		    LDFLAGS="$(LDFLAGS)"					\
 		    DESTDIR=$(OUTPUT_DIR) prefix= all install_headers
 
+$(HOST_BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile)		\
+	   $(APIDIR)/linux/bpf.h						\
+	   | $(HOST_BUILD_DIR)/libbpf
+	$(Q)$(MAKE) $(submake_extras) -C $(BPFDIR) 				\
+		    OUTPUT=$(HOST_BUILD_DIR)/libbpf/				\
+		    ARCH= CROSS_COMPILE= CC="$(HOSTCC)" LD=$(HOSTLD)		\
+		    EXTRA_CFLAGS='-g -O0 -fPIC'					\
+		    DESTDIR=$(HOST_OUTPUT_DIR) prefix= all install_headers
+
 $(DEFAULT_BPFTOOL): $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile)	\
 		    $(HOST_BPFOBJ) | $(HOST_BUILD_DIR)/bpftool
 	$(Q)$(MAKE) $(submake_extras)  -C $(BPFTOOLDIR)				\
-		    ARCH= CROSS_COMPILE= CC=$(HOSTCC) LD=$(HOSTLD)		\
+		    ARCH= CROSS_COMPILE= CC="$(HOSTCC)" LD=$(HOSTLD)		\
 		    EXTRA_CFLAGS='-g -O0'					\
 		    OUTPUT=$(HOST_BUILD_DIR)/bpftool/				\
 		    LIBBPF_OUTPUT=$(HOST_BUILD_DIR)/libbpf/			\
@@ -185,7 +196,7 @@ $(addprefix $(BINDIR)/,$(c-sched-targets)): \
 		$(SCX_COMMON_DEPS)
 	$(eval sched=$(notdir $@))
 	$(CC) $(CFLAGS) -c $(sched).c -o $(SCXOBJ_DIR)/$(sched).o
-	$(CC) -o $@ $(SCXOBJ_DIR)/$(sched).o $(HOST_BPFOBJ) $(LDFLAGS)
+	$(CC) -o $@ $(SCXOBJ_DIR)/$(sched).o $(BPFOBJ) $(LDFLAGS)
 
 $(c-sched-targets): %: $(BINDIR)/%
 
-- 
2.25.1