powerpc: Add static_key_feature_checks_initialized flag

JUMP_LABEL_FEATURE_CHECK_DEBUG used static_key_intialized to determine
whether {cpu,mmu}_has_feature() is used before static keys were
initialized. However, {cpu,mmu}_has_feature() should not be used before
setup_feature_keys() is called but static_key_initialized is set well
before this by the call to jump_label_init() in early_init_devtree().
This creates a window in which JUMP_LABEL_FEATURE_CHECK_DEBUG will not
detect misuse and report errors. Add a flag specifically to indicate
when {cpu,mmu}_has_feature() is safe to use.
Signed-off-by: Nicholas Miehlbradt <nicholas@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240408052358.5030-1-nicholas@linux.ibm.com

arch/powerpc/include/asm/cpu_has_feature.h

@@ -29,7 +29,7 @@ static __always_inline bool cpu_has_feature(unsigned long feature)
 #endif
 
 #ifdef CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG
-	if (!static_key_initialized) {
+	if (!static_key_feature_checks_initialized) {
 		printk("Warning! cpu_has_feature() used prior to jump label init!\n");
 		dump_stack();
 		return early_cpu_has_feature(feature);

arch/powerpc/include/asm/feature-fixups.h

@@ -291,6 +291,8 @@ extern long __start___rfi_flush_fixup, __stop___rfi_flush_fixup;
 extern long __start___barrier_nospec_fixup, __stop___barrier_nospec_fixup;
 extern long __start__btb_flush_fixup, __stop__btb_flush_fixup;
 
+extern bool static_key_feature_checks_initialized;
+
 void apply_feature_fixups(void);
 void update_mmu_feature_fixups(unsigned long mask);
 void setup_feature_keys(void);

arch/powerpc/include/asm/mmu.h

@@ -251,7 +251,7 @@ static __always_inline bool mmu_has_feature(unsigned long feature)
 #endif
 
 #ifdef CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG
-	if (!static_key_initialized) {
+	if (!static_key_feature_checks_initialized) {
 		printk("Warning! mmu_has_feature() used prior to jump label init!\n");
 		dump_stack();
 		return early_mmu_has_feature(feature);

arch/powerpc/lib/feature-fixups.c

@@ -25,6 +25,13 @@
 #include <asm/firmware.h>
 #include <asm/inst.h>
 
+/*
+ * Used to generate warnings if mmu or cpu feature check functions that
+ * use static keys before they are initialized.
+ */
+bool static_key_feature_checks_initialized __read_mostly;
+EXPORT_SYMBOL_GPL(static_key_feature_checks_initialized);
+
 struct fixup_entry {
 	unsigned long	mask;
 	unsigned long	value;
@@ -679,6 +686,7 @@ void __init setup_feature_keys(void)
 	jump_label_init();
 	cpu_feature_keys_init();
 	mmu_feature_keys_init();
+	static_key_feature_checks_initialized = true;
 }
 
 static int __init check_features(void)