[RFC PATCH v2 3/7] arm64: alternative: Apply alternatives early in boot process

[Daniel Thompson]

Currently alternatives are applied very late in the boot process (and
a long time after we enable scheduling). Some alternative sequences,
such as those that alter the way CPU context is stored, must be applied
much earlier in the boot sequence.

Introduce apply_alternatives_early() to allow some alternatives to be
applied immediately after we detect the CPU features of the boot CPU.

Currently apply_alternatives_all() is not optimized and will re-patch
code that has already been updated. This is harmless but could be
removed by adding extra flags to the alternatives store.

Signed-off-by: Daniel Thompson <daniel.thompson at linaro.org>
---
 arch/arm64/include/asm/alternative.h |  1 +
 arch/arm64/kernel/alternative.c      | 15 +++++++++++++++
 arch/arm64/kernel/setup.c            |  7 +++++++
 3 files changed, 23 insertions(+)

diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h
index d56ec0715157..f9dad1b7c651 100644
--- a/arch/arm64/include/asm/alternative.h
+++ b/arch/arm64/include/asm/alternative.h
@@ -17,6 +17,7 @@ struct alt_instr {
 	u8  alt_len;		/* size of new instruction(s), <= orig_len */
 };
 
+void __init apply_alternatives_early(void);
 void __init apply_alternatives_all(void);
 void apply_alternatives(void *start, size_t length);
 void free_alternatives_memory(void);
diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
index ab9db0e9818c..59989a4bed7c 100644
--- a/arch/arm64/kernel/alternative.c
+++ b/arch/arm64/kernel/alternative.c
@@ -117,6 +117,21 @@ static void __apply_alternatives(void *alt_region)
 }
 
 /*
+ * This is called very early in the boot process (directly after we run
+ * a feature detect on the boot CPU). No need to worry about other CPUs
+ * here.
+ */
+void apply_alternatives_early(void)
+{
+	struct alt_region region = {
+		.begin	= __alt_instructions,
+		.end	= __alt_instructions_end,
+	};
+
+	__apply_alternatives(&region);
+}
+
+/*
  * We might be patching the stop_machine state machine, so implement a
  * really simple polling protocol here.
  */
diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
index 6bab21f84a9f..0cddc5ff8089 100644
--- a/arch/arm64/kernel/setup.c
+++ b/arch/arm64/kernel/setup.c
@@ -211,6 +211,13 @@ static void __init setup_processor(void)
 	cpuinfo_store_boot_cpu();
 
 	/*
+	 * We now know enough about the boot CPU to apply the
+	 * alternatives that cannot wait until interrupt handling
+	 * and/or scheduling is enabled.
+	 */
+	apply_alternatives_early();
+
+	/*
 	 * Check for sane CTR_EL0.CWG value.
 	 */
 	cwg = cache_type_cwg();
-- 
2.4.3