powerpc/32: Clarify interrupt restores with REST_GPR macro in entry_32.S

Restoring the register state of the interrupted thread involves issuing
a large number of predictable loads to the kernel stack frame. Issue the
REST_GPR{,S} macros to clearly signal when this is happening, and bunch
together restores at the end of the interrupt handler where the saved
value is not consumed earlier in the handler code.

Signed-off-by: Rohan McLure <rmclure@linux.ibm.com>
Reported-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220921065605.1051927-6-rmclure@linux.ibm.com

arch/powerpc/kernel/entry_32.S

@@ -68,7 +68,7 @@ prepare_transfer_to_handler:
 	lwz	r9,_MSR(r11)		/* if sleeping, clear MSR.EE */
 	rlwinm	r9,r9,0,~MSR_EE
 	lwz	r12,_LINK(r11)		/* and return to address in LR */
-	lwz	r2, GPR2(r11)
+	REST_GPR(2, r11)
 	b	fast_exception_return
 _ASM_NOKPROBE_SYMBOL(prepare_transfer_to_handler)
 #endif /* CONFIG_PPC_BOOK3S_32 || CONFIG_PPC_E500 */
@@ -144,16 +144,16 @@ ret_from_syscall:
 	lwz	r7,_NIP(r1)
 	lwz	r8,_MSR(r1)
 	cmpwi	r3,0
-	lwz	r3,GPR3(r1)
+	REST_GPR(3, r1)
 syscall_exit_finish:
 	mtspr	SPRN_SRR0,r7
 	mtspr	SPRN_SRR1,r8
 
 	bne	3f
 	mtcr	r5
 
-1:	lwz	r2,GPR2(r1)
-	lwz	r1,GPR1(r1)
+1:	REST_GPR(2, r1)
+	REST_GPR(1, r1)
 	rfi
 #ifdef CONFIG_40x
 	b .	/* Prevent prefetch past rfi */
@@ -165,10 +165,8 @@ syscall_exit_finish:
 	REST_NVGPRS(r1)
 	mtctr	r4
 	mtxer	r5
-	lwz	r0,GPR0(r1)
-	lwz	r3,GPR3(r1)
-	REST_GPRS(4, 11, r1)
-	lwz	r12,GPR12(r1)
+	REST_GPR(0, r1)
+	REST_GPRS(3, 12, r1)
 	b	1b
 
 #ifdef CONFIG_44x
@@ -260,9 +258,8 @@ fast_exception_return:
 	beq	3f			/* if not, we've got problems */
 #endif
 
-2:	REST_GPRS(3, 6, r11)
-	lwz	r10,_CCR(r11)
-	REST_GPRS(1, 2, r11)
+2:	lwz	r10,_CCR(r11)
+	REST_GPRS(1, 6, r11)
 	mtcr	r10
 	lwz	r10,_LINK(r11)
 	mtlr	r10
@@ -277,7 +274,7 @@ fast_exception_return:
 	mtspr	SPRN_SRR0,r12
 	REST_GPR(9, r11)
 	REST_GPR(12, r11)
-	lwz	r11,GPR11(r11)
+	REST_GPR(11, r11)
 	rfi
 #ifdef CONFIG_40x
 	b .	/* Prevent prefetch past rfi */
@@ -454,9 +451,8 @@ _ASM_NOKPROBE_SYMBOL(interrupt_return)
 	lwz	r3,_MSR(r1);						\
 	andi.	r3,r3,MSR_PR;						\
 	bne	interrupt_return;					\
-	lwz	r0,GPR0(r1);						\
-	lwz	r2,GPR2(r1);						\
-	REST_GPRS(3, 8, r1);						\
+	REST_GPR(0, r1);						\
+	REST_GPRS(2, 8, r1);						\
 	lwz	r10,_XER(r1);						\
 	lwz	r11,_CTR(r1);						\
 	mtspr	SPRN_XER,r10;						\
@@ -475,11 +471,8 @@ _ASM_NOKPROBE_SYMBOL(interrupt_return)
 	lwz	r12,_MSR(r1);						\
 	mtspr	exc_lvl_srr0,r11;					\
 	mtspr	exc_lvl_srr1,r12;					\
-	lwz	r9,GPR9(r1);						\
-	lwz	r12,GPR12(r1);						\
-	lwz	r10,GPR10(r1);						\
-	lwz	r11,GPR11(r1);						\
-	lwz	r1,GPR1(r1);						\
+	REST_GPRS(9, 12, r1);						\
+	REST_GPR(1, r1);						\
 	exc_lvl_rfi;							\
 	b	.;		/* prevent prefetch past exc_lvl_rfi */