Enable LLVM toolchain for building LKL targets.

To build LKL targets with clang/ld.lld provide LLVM and CROSS_COMPILE
variables on the make command line in a similar way they used in the
upstream Linux mainline:

// to make LKL targets with the default versions of clang/ld.lld
make -C tools/lkl LLVM=1 CROSS_COMPILE=x86_64-linux-gnu

// to make LKL targets with clang/ld.lld in a specific location
make -C tools/lkl LLVM=/usr/bin/ CROSS_COMPILE=x86_64-linux-gnu

// to make LKL targets with a specific versions of clang/ld.lld, e.g. 13
make -C tools/lkl LLVM=-13 CROSS_COMPILE=x86_64-linux-gnu

Additional documentation on LLVM/CROSS_COMPILE variables is available in
Documentation/kbuild/llvm.rst.

At the moment the LKL makefiles support x86_64-linux-gnu LLVM target
only.

Signed-off-by: Eugene Rodionov <[email protected]>

arch/lkl/kernel/misc.c

@@ -1,5 +1,23 @@
 #include <linux/seq_file.h>
 
+// TODO: Functions __generic_xchg_called_with_bad_pointer and wrong_size_cmpxchg
+// are called in __generic_xchg and __generic_cmpxchg_local respectively.
+// They should be optimized out by the compiler due to the function
+// inlining. However, when building with clang there are some instances
+// where the functions aren't inlined and, thus, the compile-time optimization
+// doesn't eliminate them entirely. As a result, the linker throws
+// unresololved symbols error. As a workaround, the fix below define these
+// functions to bypass the link-time error.
+void __generic_xchg_called_with_bad_pointer(void)
+{
+	panic("%s shouldn't be executed\n", __func__);
+}
+unsigned long wrong_size_cmpxchg(volatile void *ptr)
+{
+	panic("%s shouldn't be executed\n", __func__);
+	return 0;
+}
+
 #ifdef CONFIG_PROC_FS
 static void *cpuinfo_start(struct seq_file *m, loff_t *pos)
 {

tools/lkl/Makefile

@@ -105,6 +105,7 @@ $(OUTPUT)cpfromfs$(EXESUF): cptofs$(EXESUF)
 	$(Q)if ! [ -e $@ ]; then ln -s $< $@; fi
 
 clean:
+	$(call QUIET_CLEAN, vmlinux)$(MAKE) -C ../.. ARCH=lkl $(KOPT) clean
 	$(call QUIET_CLEAN, objects)find $(OUTPUT) -name '*.o' -delete -o -name '\.*.cmd'\
 	 -delete -o -name '\.*.d' -delete
 	$(call QUIET_CLEAN, headers)$(RM) -r $(OUTPUT)/include/lkl/
@@ -113,6 +114,8 @@ clean:
 
 clean-conf: clean
 	$(call QUIET_CLEAN, Makefile.conf)$(RM) $(OUTPUT)/Makefile.conf
+	$(call QUIET_CLEAN, kernel_config.h)$(RM) $(OUTPUT)/include/kernel_config.h
+	$(call QUIET_CLEAN, kernel.config)$(RM) $(OUTPUT)/kernel.config
 
 headers_install: $(TARGETS)
 	$(call QUIET_INSTALL, headers) \

tools/lkl/Makefile.autoconf

@@ -142,14 +142,35 @@ define kasan_enable
   $(if $(filter yes,$(kasan_test)), $(call kasan_test_enable))
 endef
 
-define do_autoconf
+define do_autoconf_gnu
   export CROSS_COMPILE := $(CROSS_COMPILE)
   export CC := $(CROSS_COMPILE)gcc
   export LD := $(CROSS_COMPILE)ld
   export AR := $(CROSS_COMPILE)ar
   $(eval LD := $(CROSS_COMPILE)ld)
   $(eval CC := $(CROSS_COMPILE)gcc)
   $(eval LD_FMT := $(shell $(LD) -r -print-output-format))
+endef
+
+define llvm_target_to_ld_fmt
+  $(if $(filter $(CROSS_COMPILE),x86_64-linux-gnu),elf64-x86-64,\
+    $(error Unsupported LLVM target $(CROSS_COMPILE)))
+endef
+
+define do_autoconf_llvm
+  $(eval LLVM_PREFIX := $(if $(filter %/,$(LLVM)),$(LLVM)))
+  $(eval LLVM_SUFFIX := $(if $(filter -%,$(LLVM)),$(LLVM)))
+  export CROSS_COMPILE := $(CROSS_COMPILE)
+  export CC := $(LLVM_PREFIX)clang$(LLVM_SUFFIX)
+  export LD := $(LLVM_PREFIX)ld.lld$(LLVM_SUFFIX)
+  export AR := $(LLVM_PREFIX)llvm-ar$(LLVM_SUFFIX)
+  $(eval LD := $(LLVM_PREFIX)ld.lld$(LLVM_SUFFIX))
+  $(eval CC := $(LLVM_PREFIX)clang$(LLVM_SUFFIX))
+  $(eval LD_FMT := $(call llvm_target_to_ld_fmt))
+endef
+
+define do_autoconf
+  $(if $(LLVM),$(call do_autoconf_llvm),$(call do_autoconf_gnu))
   $(eval EXEC_FMT := $(shell echo $(LD_FMT) | cut -d "-" -f1))
   $(call set_kernel_config,OUTPUT_FORMAT,\"$(LD_FMT)\")
   $(if $(or $(filter $(EXEC_FMT),elf64),$(filter $(LD_FMT),pe-x86-64)),$(call 64bit_host))