powerpc/64: Don't trace code that runs with the soft irq mask unreconciled

"Reconciling" in terms of interrupt handling, is to bring the soft irq
mask state in to synch with the hardware, after an interrupt causes
MSR[EE] to be cleared (while the soft mask may be enabled, and hard
irqs not marked disabled).

General kernel code should not be called while unreconciled, because
local_irq_disable, etc. manipulations can cause surprising irq traces,
and it's fragile because the soft irq code does not really expect to
be called in this situation.

When exiting from an interrupt, MSR[EE] is cleared to prevent races,
but soft irq state is enabled for the returned-to context, so this is
now an unreconciled state. restore_math is called in this state, and
that can be ftraced, and the ftrace subsystem disables local irqs.

Mark restore_math and its callees as notrace. Restore a sanity check
in the soft irq code that had to be disabled for this case, by commit
4da1f792 ("powerpc/64: Disable irq restore warning for now").
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/fpu.S

@@ -63,6 +63,7 @@ _GLOBAL(load_fp_state)
 	REST_32FPVSRS(0, R4, R3)
 	blr
 EXPORT_SYMBOL(load_fp_state)
+_ASM_NOKPROBE_SYMBOL(load_fp_state); /* used by restore_math */
 
 /*
  * Store FP state into memory, including FPSCR

arch/powerpc/kernel/irq.c

@@ -258,16 +258,9 @@ notrace void arch_local_irq_restore(unsigned long mask)
 	 */
 	irq_happened = get_irq_happened();
 	if (!irq_happened) {
-		/*
-		 * FIXME. Here we'd like to be able to do:
-		 *
-		 * #ifdef CONFIG_PPC_IRQ_SOFT_MASK_DEBUG
-		 *   WARN_ON(!(mfmsr() & MSR_EE));
-		 * #endif
-		 *
-		 * But currently it hits in a few paths, we should fix those and
-		 * enable the warning.
-		 */
+#ifdef CONFIG_PPC_IRQ_SOFT_MASK_DEBUG
+		WARN_ON(!(mfmsr() & MSR_EE));
+#endif
 		return;
 	}
 

arch/powerpc/kernel/process.c

@@ -134,7 +134,8 @@ static int __init enable_strict_msr_control(char *str)
 }
 early_param("ppc_strict_facility_enable", enable_strict_msr_control);
 
-unsigned long msr_check_and_set(unsigned long bits)
+/* notrace because it's called by restore_math */
+unsigned long notrace msr_check_and_set(unsigned long bits)
 {
 	unsigned long oldmsr = mfmsr();
 	unsigned long newmsr;
@@ -153,7 +154,8 @@ unsigned long msr_check_and_set(unsigned long bits)
 }
 EXPORT_SYMBOL_GPL(msr_check_and_set);
 
-void __msr_check_and_clear(unsigned long bits)
+/* notrace because it's called by restore_math */
+void notrace __msr_check_and_clear(unsigned long bits)
 {
 	unsigned long oldmsr = mfmsr();
 	unsigned long newmsr;
@@ -526,7 +528,17 @@ void giveup_all(struct task_struct *tsk)
 }
 EXPORT_SYMBOL(giveup_all);
 
-void restore_math(struct pt_regs *regs)
+/*
+ * The exception exit path calls restore_math() with interrupts hard disabled
+ * but the soft irq state not "reconciled". ftrace code that calls
+ * local_irq_save/restore causes warnings.
+ *
+ * Rather than complicate the exit path, just don't trace restore_math. This
+ * could be done by having ftrace entry code check for this un-reconciled
+ * condition where MSR[EE]=0 and PACA_IRQ_HARD_DIS is not set, and
+ * temporarily fix it up for the duration of the ftrace call.
+ */
+void notrace restore_math(struct pt_regs *regs)
 {
 	unsigned long msr;
 

arch/powerpc/kernel/vector.S

@@ -21,6 +21,7 @@ _GLOBAL(load_vr_state)
 	REST_32VRS(0,r4,r3)
 	blr
 EXPORT_SYMBOL(load_vr_state)
+_ASM_NOKPROBE_SYMBOL(load_vr_state); /* used by restore_math */
 
 /*
  * Store VMX state into memory, including VSCR.