powerpc/64s/kuap: Conditionally restore AMR in kuap_restore_amr asm

Similar to the C code change, make the AMR restore conditional on
whether the register has changed.
Signed-off-by: NNicholas Piggin <npiggin@gmail.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200429065654.1677541-7-npiggin@gmail.com

arch/powerpc/include/asm/book3s/64/kup-radix.h

@@ -12,13 +12,17 @@
 
 #ifdef __ASSEMBLY__
 
-.macro kuap_restore_amr	gpr
+.macro kuap_restore_amr	gpr1, gpr2
 #ifdef CONFIG_PPC_KUAP
 	BEGIN_MMU_FTR_SECTION_NESTED(67)
-	ld	\gpr, STACK_REGS_KUAP(r1)
+	mfspr	\gpr1, SPRN_AMR
+	ld	\gpr2, STACK_REGS_KUAP(r1)
+	cmpd	\gpr1, \gpr2
+	beq	998f
 	isync
-	mtspr	SPRN_AMR, \gpr
+	mtspr	SPRN_AMR, \gpr2
 	/* No isync required, see kuap_restore_amr() */
+998:
 	END_MMU_FTR_SECTION_NESTED_IFSET(MMU_FTR_RADIX_KUAP, 67)
 #endif
 .endm

arch/powerpc/kernel/entry_64.S

@@ -479,11 +479,11 @@ END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_207S)
 fast_interrupt_return:
 _ASM_NOKPROBE_SYMBOL(fast_interrupt_return)
 	kuap_check_amr r3, r4
-	ld	r4,_MSR(r1)
-	andi.	r0,r4,MSR_PR
+	ld	r5,_MSR(r1)
+	andi.	r0,r5,MSR_PR
 	bne	.Lfast_user_interrupt_return
-	kuap_restore_amr r3
-	andi.	r0,r4,MSR_RI
+	kuap_restore_amr r3, r4
+	andi.	r0,r5,MSR_RI
 	li	r3,0 /* 0 return value, no EMULATE_STACK_STORE */
 	bne+	.Lfast_kernel_interrupt_return
 	addi	r3,r1,STACK_FRAME_OVERHEAD

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

@@ -971,7 +971,7 @@ EXC_COMMON_BEGIN(system_reset_common)
 	ld	r10,SOFTE(r1)
 	stb	r10,PACAIRQSOFTMASK(r13)
 
-	kuap_restore_amr r10
+	kuap_restore_amr r9, r10
 	EXCEPTION_RESTORE_REGS
 	RFI_TO_USER_OR_KERNEL
 
@@ -2784,7 +2784,7 @@ EXC_COMMON_BEGIN(soft_nmi_common)
 	ld	r10,SOFTE(r1)
 	stb	r10,PACAIRQSOFTMASK(r13)
 
-	kuap_restore_amr r10
+	kuap_restore_amr r9, r10
 	EXCEPTION_RESTORE_REGS hsrr=0
 	RFI_TO_KERNEL