powerpc/64: treat low kernel text as irqs soft-masked

Treat code below __end_soft_masked as soft-masked for the purpose
of alternate return. 64s already mostly does this for scv entry.

This will be used to exit from interrupts without disabling MSR[EE].
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210617155116.2167984-12-npiggin@gmail.com

arch/powerpc/include/asm/interrupt.h

@@ -146,8 +146,13 @@ static inline void interrupt_enter_prepare(struct pt_regs *regs, struct interrup
 		 * CT_WARN_ON comes here via program_check_exception,
 		 * so avoid recursion.
 		 */
-		if (TRAP(regs) != INTERRUPT_PROGRAM)
+		if (TRAP(regs) != INTERRUPT_PROGRAM) {
 			CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
+			BUG_ON(regs->nip < (unsigned long)__end_soft_masked);
+		}
+		/* Move this under a debugging check */
+		if (arch_irq_disabled_regs(regs))
+			BUG_ON(search_kernel_restart_table(regs->nip));
 	}
 #endif
 
@@ -238,8 +243,8 @@ static inline void interrupt_nmi_enter_prepare(struct pt_regs *regs, struct inte
 	local_paca->irq_happened |= PACA_IRQ_HARD_DIS;
 
 	if (IS_ENABLED(CONFIG_PPC_BOOK3S_64) && !(regs->msr & MSR_PR) &&
-				regs->nip < (unsigned long)__end_interrupts) {
-		// Kernel code running below __end_interrupts is
+				regs->nip < (unsigned long)__end_soft_masked) {
+		// Kernel code running below __end_soft_masked is
 		// implicitly soft-masked.
 		regs->softe = IRQS_ALL_DISABLED;
 	}

arch/powerpc/kernel/exceptions-64e.S

@@ -342,7 +342,17 @@ ret_from_mc_except:
 #define PROLOG_ADDITION_MASKABLE_GEN(n)					    \
 	lbz	r10,PACAIRQSOFTMASK(r13);	/* are irqs soft-masked? */ \
 	andi.	r10,r10,IRQS_DISABLED;	/* yes -> go out of line */ \
-	bne	masked_interrupt_book3e_##n
+	bne	masked_interrupt_book3e_##n;				    \
+	/* Kernel code below __end_soft_masked is implicitly masked */	    \
+	andi.	r10,r11,MSR_PR;						    \
+	bne	1f;			/* user -> not masked */	    \
+	std	r14,PACA_EXGEN+EX_R14(r13);				    \
+	LOAD_REG_IMMEDIATE_SYM(r14, r10, __end_soft_masked);		    \
+	mfspr	r10,SPRN_SRR0;						    \
+	cmpld	r10,r14;						    \
+	ld	r14,PACA_EXGEN+EX_R14(r13);				    \
+	blt	masked_interrupt_book3e_##n;				    \
+1:
 
 /*
  * Additional regs must be re-loaded from paca before EXCEPTION_COMMON* is

arch/powerpc/kernel/exceptions-64s.S

@@ -430,10 +430,13 @@ DEFINE_FIXED_SYMBOL(\name\()_common_real)
 		andi.	r10,r12,MSR_PR
 		bne	2f
 
-		/* Kernel code running below __end_interrupts is implicitly
-		 * soft-masked */
-		LOAD_HANDLER(r10, __end_interrupts)
+		/*
+		 * Kernel code running below __end_soft_masked is implicitly
+		 * soft-masked
+		 */
+		LOAD_HANDLER(r10, __end_soft_masked)
 		cmpld	r11,r10
+
 		li	r10,IMASK
 		blt-	1f
 
@@ -751,17 +754,17 @@ __start_interrupts:
  * scv instructions enter the kernel without changing EE, RI, ME, or HV.
  * In particular, this means we can take a maskable interrupt at any point
  * in the scv handler, which is unlike any other interrupt. This is solved
- * by treating the instruction addresses below __end_interrupts as being
+ * by treating the instruction addresses below __end_soft_masked as being
  * soft-masked.
  *
  * AIL-0 mode scv exceptions go to 0x17000-0x17fff, but we set AIL-3 and
  * ensure scv is never executed with relocation off, which means AIL-0
  * should never happen.
  *
- * Before leaving the below __end_interrupts text, at least of the following
- * must be true:
+ * Before leaving the following inside-__end_soft_masked text, at least of the
+ * following must be true:
  * - MSR[PR]=1 (i.e., return to userspace)
- * - MSR_EE|MSR_RI is set (no reentrant exceptions)
+ * - MSR_EE|MSR_RI is clear (no reentrant exceptions)
  * - Standard kernel environment is set up (stack, paca, etc)
  *
  * Call convention:
@@ -2957,7 +2960,7 @@ MASKED_INTERRUPT hsrr=1
 
 USE_FIXED_SECTION(virt_trampolines)
 	/*
-	 * All code below __end_interrupts is treated as soft-masked. If
+	 * All code below __end_soft_masked is treated as soft-masked. If
 	 * any code runs here with MSR[EE]=1, it must then cope with pending
 	 * soft interrupt being raised (i.e., by ensuring it is replayed).
 	 *

arch/powerpc/kernel/interrupt_64.S

@@ -632,4 +632,8 @@ ALT_FTR_SECTION_END_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
 interrupt_return_macro srr
 #ifdef CONFIG_PPC_BOOK3S
 interrupt_return_macro hsrr
-#endif
+#endif /* CONFIG_PPC_BOOK3S */
+
+	.globl __end_soft_masked
+__end_soft_masked:
+DEFINE_FIXED_SYMBOL(__end_soft_masked)