[PATCH 2/3] x86/boot: avoid recompiling misc.c for incremental rebuilds

[Jann Horn <jannh@google.com>]

Currently, we rebuild misc.c on every incremental compilation because
the generated header voffset.h (containing kernel image offsets)
changes on every build.

Turn the three macros we need from that header into external variables
that we can separately store in another C file.

Signed-off-by: Jann Horn <jannh@google.com>
---
 arch/x86/boot/compressed/Makefile       |  4 ++--
 arch/x86/boot/compressed/dynamic_vars.c |  9 +++++++++
 arch/x86/boot/compressed/dynamic_vars.h | 11 +++++++++++
 arch/x86/boot/compressed/misc.c         |  6 ++----
 4 files changed, 24 insertions(+), 6 deletions(-)
 create mode 100644 arch/x86/boot/compressed/dynamic_vars.c
 create mode 100644 arch/x86/boot/compressed/dynamic_vars.h

diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index f19c038409aa..d18a553c16fa 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -94,11 +94,11 @@ targets += ../voffset.h
 $(obj)/../voffset.h: vmlinux FORCE
 	$(call if_changed,voffset)
 
-$(obj)/misc.o: $(obj)/../voffset.h
+$(obj)/dynamic_vars.o: $(obj)/../voffset.h
 
 vmlinux-objs-y := $(obj)/vmlinux.lds $(obj)/kernel_info.o $(obj)/head_$(BITS).o \
 	$(obj)/misc.o $(obj)/string.o $(obj)/cmdline.o $(obj)/error.o \
-	$(obj)/piggy.o $(obj)/cpuflags.o
+	$(obj)/piggy.o $(obj)/cpuflags.o $(obj)/dynamic_vars.o
 
 vmlinux-objs-$(CONFIG_EARLY_PRINTK) += $(obj)/early_serial_console.o
 vmlinux-objs-$(CONFIG_RANDOMIZE_BASE) += $(obj)/kaslr.o
diff --git a/arch/x86/boot/compressed/dynamic_vars.c b/arch/x86/boot/compressed/dynamic_vars.c
new file mode 100644
index 000000000000..cda64ff4b6da
--- /dev/null
+++ b/arch/x86/boot/compressed/dynamic_vars.c
@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/const.h>
+#include "dynamic_vars.h"
+#include "../voffset.h"
+
+const unsigned long vo__text = VO__text;
+const unsigned long vo___bss_start = VO___bss_start;
+const unsigned long vo__end = VO__end;
+const unsigned long kernel_total_size = VO__end - VO__text;
diff --git a/arch/x86/boot/compressed/dynamic_vars.h b/arch/x86/boot/compressed/dynamic_vars.h
new file mode 100644
index 000000000000..a0f7dc359cb6
--- /dev/null
+++ b/arch/x86/boot/compressed/dynamic_vars.h
@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * This header defines some variables that change on every compilation and are
+ * stored separately in the small file dynamic-vars.c, so that we can avoid
+ * rebuilding some of the other C files in this directory on every incremental
+ * rebuild.
+ */
+
+/* Variables containing VO__text, VO___bss_start, VO__end */
+extern const unsigned long vo__text, vo___bss_start, vo__end;
+extern const unsigned long kernel_total_size;
diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index b99e08e6815b..ff13cc3e703d 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -15,8 +15,8 @@
 #include "misc.h"
 #include "error.h"
 #include "pgtable.h"
+#include "dynamic_vars.h"
 #include "../string.h"
-#include "../voffset.h"
 #include <asm/bootparam_utils.h>
 
 /*
@@ -188,7 +188,7 @@ static void handle_relocations(void *output, unsigned long output_len,
 	int *reloc;
 	unsigned long delta, map, ptr;
 	unsigned long min_addr = (unsigned long)output;
-	unsigned long max_addr = min_addr + (VO___bss_start - VO__text);
+	unsigned long max_addr = min_addr + (vo___bss_start - vo__text);
 
 	/*
 	 * Calculate the delta between where vmlinux was linked to load
@@ -330,8 +330,6 @@ static size_t parse_elf(void *output)
 	return ehdr.e_entry - LOAD_PHYSICAL_ADDR;
 }
 
-const unsigned long kernel_total_size = VO__end - VO__text;
-
 static u8 boot_heap[BOOT_HEAP_SIZE] __aligned(4);
 
 extern unsigned char input_data[];
-- 
2.44.0.rc0.258.g7320e95886-goog