[PATCH v2] powerpc/64: handle linker stubs in low .text code

[Nicholas Piggin]

Very large kernels may require linker stubs for branches from HEAD
text code. The linker may place these stubs before the HEAD text
sections, which breaks the assumption that HEAD text is located at 0
(or the .text section being located at 0x7000/0x8000 on Book3S
kernels).

Provide an option to create a small section just before the .text
section with an empty 256 - 4 bytes, and adjust the start of the .text
section to match. The linker will tend to put stubs in that section
and not break our relative-to-absolute offset assumptions.

This causes a small waste of space on common kernels, but allows large
kernels to build and boot. For now, it is an EXPERT config option,
defaulting to =n, but a reference is provided for it in the build-time
check for such breakage. This is good enough for allyesconfig and
custom users / hackers.

Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
---
 arch/powerpc/Kconfig               | 11 +++++++++++
 arch/powerpc/include/asm/head-64.h | 18 ++++++++++++++++++
 arch/powerpc/kernel/vmlinux.lds.S  |  5 +++++
 arch/powerpc/tools/head_check.sh   |  7 +++++++
 4 files changed, 41 insertions(+)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index f7c8f9972f61..6eb70c96ec5e 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -454,6 +454,17 @@ config PPC_TRANSACTIONAL_MEM
        ---help---
          Support user-mode Transactional Memory on POWERPC.
 
+config LD_HEAD_STUB_CATCH
+	bool "Reserve 256 bytes to cope with linker stubs in HEAD text" if EXPERT
+	depends on PPC64
+	default n
+	help
+	  Very large kernels can cause linker branch stubs to be generated by
+	  code in head_64.S, which moves the head text sections out of their
+	  specified location. This option can work around the problem.
+
+	  If unsure, say "N".
+
 config DISABLE_MPROFILE_KERNEL
 	bool "Disable use of mprofile-kernel for kernel tracing"
 	depends on PPC64 && CPU_LITTLE_ENDIAN
diff --git a/arch/powerpc/include/asm/head-64.h b/arch/powerpc/include/asm/head-64.h
index 99c9f01d02df..7ab95798f170 100644
--- a/arch/powerpc/include/asm/head-64.h
+++ b/arch/powerpc/include/asm/head-64.h
@@ -63,11 +63,29 @@
 	. = 0x0;						\
 start_##sname:
 
+/*
+ * .linker_stub_catch section is used to catch linker stubs from being
+ * inserted in our .text section, above the start_text label (which breaks
+ * the ABS_ADDR calculation). See kernel/vmlinux.lds.S and tools/head_check.sh
+ * for more details. We would prefer to just keep a cacheline (0x80), but
+ * 0x100 seems to be how the linker aligns branch stub groups.
+ */
+#ifdef CONFIG_LD_HEAD_STUB_CATCH
+#define OPEN_TEXT_SECTION(start)				\
+	.section ".linker_stub_catch","ax", at progbits;		\
+linker_stub_catch:						\
+	. = 0x4;						\
+	text_start = (start) + 0x100;				\
+	.section ".text","ax", at progbits;			\
+	.balign 0x100;						\
+start_text:
+#else
 #define OPEN_TEXT_SECTION(start)				\
 	text_start = (start);					\
 	.section ".text","ax", at progbits;			\
 	. = 0x0;						\
 start_text:
+#endif
 
 #define ZERO_FIXED_SECTION(sname, start, end)			\
 	sname##_start = (start);				\
diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index 5aa434e84605..e69155f0db36 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S
@@ -81,6 +81,11 @@ SECTIONS
 	 * section placement to work.
 	 */
 	.text BLOCK(0) : AT(ADDR(.text) - LOAD_OFFSET) {
+#ifdef CONFIG_LD_HEAD_STUB_CATCH
+		*(.linker_stub_catch);
+		. = . ;
+#endif
+
 #else
 	.text : AT(ADDR(.text) - LOAD_OFFSET) {
 		ALIGN_FUNCTION();
diff --git a/arch/powerpc/tools/head_check.sh b/arch/powerpc/tools/head_check.sh
index bfde937564f9..ad9e57209aa4 100755
--- a/arch/powerpc/tools/head_check.sh
+++ b/arch/powerpc/tools/head_check.sh
@@ -21,6 +21,11 @@
 # fixed section region (0 - 0x8000ish). Check what code is calling those stubs,
 # and perhaps change so a direct branch can reach.
 #
+# A ".linker_stub_catch" section is used to catch some stubs generated by
+# early .text code, which tend to get placed at the start of the section.
+# If there are too many such stubs, they can overflow this section. Expanding
+# it may help (or reducing the number of stub branches).
+#
 # Linker stubs use the TOC pointer, so even if fixed section code could
 # tolerate them being inserted into head code, they can't be allowed in low
 # level entry code (boot, interrupt vectors, etc) until r2 is set up. This
@@ -50,6 +55,7 @@ start_head_addr=$(cat .tmp_symbols.txt | grep " t start_first_256B$" | cut -d' '
 
 if [ "$start_head_addr" != "$expected_start_head_addr" ]; then
 	echo "ERROR: head code starts at $start_head_addr, should be $expected_start_head_addr"
+	echo "ERROR: try to enable LD_HEAD_STUB_CATCH config option"
 	echo "ERROR: see comments in arch/powerpc/tools/head_check.sh"
 
 	exit 1
@@ -63,6 +69,7 @@ start_text_addr=$(cat .tmp_symbols.txt | grep " t start_text$" | cut -d' ' -f1)
 
 if [ "$start_text_addr" != "$expected_start_text_addr" ]; then
 	echo "ERROR: start_text address is $start_text_addr, should be $expected_start_text_addr"
+	echo "ERROR: try to enable LD_HEAD_STUB_CATCH config option"
 	echo "ERROR: see comments in arch/powerpc/tools/head_check.sh"
 
 	exit 1
-- 
2.11.0