[RFC PATCH 7/8] powerpc/32: use IRQ stack immediately on IRQ exception

[Christophe Leroy]

Exception entries run of kernel thread stack, then do_IRQ()
switches to the IRQ stack.

Instead of doing a first step of the thread stack, increasing the
risk of stack overflow and spending time switch stacks two times when
coming from userspace, set the stack to IRQ stack immediately in the
EXCEPTION entry.

In the same way as ARM64, consider that when the stack pointer is not
within the kernel thread stack, it means it is already on IRQ stack.

Signed-off-by: Christophe Leroy <christophe.leroy at c-s.fr>
---
 arch/powerpc/kernel/head_32.S  |  2 +-
 arch/powerpc/kernel/head_32.h  | 32 +++++++++++++++++++++++++++++---
 arch/powerpc/kernel/head_40x.S |  2 +-
 arch/powerpc/kernel/head_8xx.S |  2 +-
 4 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/kernel/head_32.S b/arch/powerpc/kernel/head_32.S
index 4a24f8f026c7..0c36fba5b861 100644
--- a/arch/powerpc/kernel/head_32.S
+++ b/arch/powerpc/kernel/head_32.S
@@ -332,7 +332,7 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_HPTE_TABLE)
 	EXC_XFER_LITE(0x400, handle_page_fault)
 
 /* External interrupt */
-	EXCEPTION(0x500, HardwareInterrupt, do_IRQ, EXC_XFER_LITE)
+	EXCEPTION_IRQ(0x500, HardwareInterrupt, __do_irq, EXC_XFER_LITE)
 
 /* Alignment exception */
 	. = 0x600
diff --git a/arch/powerpc/kernel/head_32.h b/arch/powerpc/kernel/head_32.h
index 8abc7783dbe5..f9e77e51723e 100644
--- a/arch/powerpc/kernel/head_32.h
+++ b/arch/powerpc/kernel/head_32.h
@@ -11,21 +11,41 @@
  * task's thread_struct.
  */
 
-.macro EXCEPTION_PROLOG
+.macro EXCEPTION_PROLOG is_irq=0
 	mtspr	SPRN_SPRG_SCRATCH0,r10
 	mtspr	SPRN_SPRG_SCRATCH1,r11
 	mfcr	r10
-	EXCEPTION_PROLOG_1
+	EXCEPTION_PROLOG_1 is_irq=\is_irq
 	EXCEPTION_PROLOG_2
 .endm
 
-.macro EXCEPTION_PROLOG_1
+.macro EXCEPTION_PROLOG_1 is_irq=0
 	mfspr	r11,SPRN_SRR1		/* check whether user or kernel */
 	andi.	r11,r11,MSR_PR
+	.if \is_irq
+	bne	2f
+	mfspr	r11, SPRN_SPRG_THREAD
+	lwz	r11, TASK_STACK - THREAD(r11)
+	xor	r11, r11, r1
+	cmplwi	cr7, r11, THREAD_SIZE - 1
+	tophys(r11, r1)			/* use tophys(r1) if not thread stack */
+	bgt	cr7, 1f
+2:
+#ifdef CONFIG_SMP
+	mfspr	r11, SPRN_SPRG_THREAD
+	lwz	r11, TASK_CPU - THREAD(r11)
+	slwi	r11, r11, 3
+	addis	r11, r11, (hardirq_ctx - PAGE_OFFSET)@ha
+#else
+	lis	r11, (hardirq_ctx - PAGE_OFFSET)@ha
+#endif
+	lwz	r11, (hardirq_ctx - PAGE_OFFSET)@l(r11)
+	.else
 	tophys(r11,r1)			/* use tophys(r1) if kernel */
 	beq	1f
 	mfspr	r11,SPRN_SPRG_THREAD
 	lwz	r11,TASK_STACK-THREAD(r11)
+	.endif
 	addi	r11,r11,THREAD_SIZE
 	tophys(r11,r11)
 1:	subi	r11,r11,INT_FRAME_SIZE	/* alloc exc. frame */
@@ -171,6 +191,12 @@
 	addi	r3,r1,STACK_FRAME_OVERHEAD;	\
 	xfer(n, hdlr)
 
+#define EXCEPTION_IRQ(n, label, hdlr, xfer)	\
+	START_EXCEPTION(n, label)		\
+	EXCEPTION_PROLOG is_irq=1;		\
+	addi	r3,r1,STACK_FRAME_OVERHEAD;	\
+	xfer(n, hdlr)
+
 #define EXC_XFER_TEMPLATE(hdlr, trap, msr, tfer, ret)		\
 	li	r10,trap;					\
 	stw	r10,_TRAP(r11);					\
diff --git a/arch/powerpc/kernel/head_40x.S b/arch/powerpc/kernel/head_40x.S
index 4511fc1549f7..dd236f596c0b 100644
--- a/arch/powerpc/kernel/head_40x.S
+++ b/arch/powerpc/kernel/head_40x.S
@@ -315,7 +315,7 @@ _ENTRY(crit_srr1)
 	EXC_XFER_LITE(0x400, handle_page_fault)
 
 /* 0x0500 - External Interrupt Exception */
-	EXCEPTION(0x0500, HardwareInterrupt, do_IRQ, EXC_XFER_LITE)
+	EXCEPTION_IRQ(0x0500, HardwareInterrupt, __do_irq, EXC_XFER_LITE)
 
 /* 0x0600 - Alignment Exception */
 	START_EXCEPTION(0x0600, Alignment)
diff --git a/arch/powerpc/kernel/head_8xx.S b/arch/powerpc/kernel/head_8xx.S
index 19f583e18402..5a6cdbc89e26 100644
--- a/arch/powerpc/kernel/head_8xx.S
+++ b/arch/powerpc/kernel/head_8xx.S
@@ -150,7 +150,7 @@ DataAccess:
 InstructionAccess:
 
 /* External interrupt */
-	EXCEPTION(0x500, HardwareInterrupt, do_IRQ, EXC_XFER_LITE)
+	EXCEPTION_IRQ(0x500, HardwareInterrupt, __do_irq, EXC_XFER_LITE)
 
 /* Alignment exception */
 	. = 0x600
-- 
2.13.3