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Message-Id: <f46a01750b1a00c9c43725899c9cf8eb6c6a0587.1613025208.git.christophe.leroy@csgroup.eu>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Subject: [PATCH] powerpc/traps: Declare unrecoverable_exception() as
 __noreturn
To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>, npiggin@gmail.com
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Date:   Thu, 11 Feb 2021 06:34:43 +0000 (UTC)
Precedence: bulk

unrecoverable_exception() is never expected to return, most callers
have an infiniteloop in case it returns.

Ensure it really never returns by terminating it with a BUG(), and
declare it __no_return.

It always GCC to really simplify functions calling it. In the exemple below,
it avoids the stack frame in the likely fast path and avoids code duplication
for the exit.

With this patch:

	00000348 <interrupt_exit_kernel_prepare>:
	 348:	81 43 00 84 	lwz     r10,132(r3)
	 34c:	71 48 00 02 	andi.   r8,r10,2
	 350:	41 82 00 2c 	beq     37c <interrupt_exit_kernel_prepare+0x34>
	 354:	71 4a 40 00 	andi.   r10,r10,16384
	 358:	40 82 00 20 	bne     378 <interrupt_exit_kernel_prepare+0x30>
	 35c:	80 62 00 70 	lwz     r3,112(r2)
	 360:	74 63 00 01 	andis.  r3,r3,1
	 364:	40 82 00 28 	bne     38c <interrupt_exit_kernel_prepare+0x44>
	 368:	7d 40 00 a6 	mfmsr   r10
	 36c:	7c 11 13 a6 	mtspr   81,r0
	 370:	7c 12 13 a6 	mtspr   82,r0
	 374:	4e 80 00 20 	blr
	 378:	48 00 00 00 	b       378 <interrupt_exit_kernel_prepare+0x30>
	 37c:	94 21 ff f0 	stwu    r1,-16(r1)
	 380:	7c 08 02 a6 	mflr    r0
	 384:	90 01 00 14 	stw     r0,20(r1)
	 388:	48 00 00 01 	bl      388 <interrupt_exit_kernel_prepare+0x40>
				388: R_PPC_REL24	unrecoverable_exception
	 38c:	38 e2 00 70 	addi    r7,r2,112
	 390:	3d 00 00 01 	lis     r8,1
	 394:	7c c0 38 28 	lwarx   r6,0,r7
	 398:	7c c6 40 78 	andc    r6,r6,r8
	 39c:	7c c0 39 2d 	stwcx.  r6,0,r7
	 3a0:	40 a2 ff f4 	bne     394 <interrupt_exit_kernel_prepare+0x4c>
	 3a4:	38 60 00 01 	li      r3,1
	 3a8:	4b ff ff c0 	b       368 <interrupt_exit_kernel_prepare+0x20>

Without this patch:

	00000348 <interrupt_exit_kernel_prepare>:
	 348:	94 21 ff f0 	stwu    r1,-16(r1)
	 34c:	93 e1 00 0c 	stw     r31,12(r1)
	 350:	7c 7f 1b 78 	mr      r31,r3
	 354:	81 23 00 84 	lwz     r9,132(r3)
	 358:	71 2a 00 02 	andi.   r10,r9,2
	 35c:	41 82 00 34 	beq     390 <interrupt_exit_kernel_prepare+0x48>
	 360:	71 29 40 00 	andi.   r9,r9,16384
	 364:	40 82 00 28 	bne     38c <interrupt_exit_kernel_prepare+0x44>
	 368:	80 62 00 70 	lwz     r3,112(r2)
	 36c:	74 63 00 01 	andis.  r3,r3,1
	 370:	40 82 00 3c 	bne     3ac <interrupt_exit_kernel_prepare+0x64>
	 374:	7d 20 00 a6 	mfmsr   r9
	 378:	7c 11 13 a6 	mtspr   81,r0
	 37c:	7c 12 13 a6 	mtspr   82,r0
	 380:	83 e1 00 0c 	lwz     r31,12(r1)
	 384:	38 21 00 10 	addi    r1,r1,16
	 388:	4e 80 00 20 	blr
	 38c:	48 00 00 00 	b       38c <interrupt_exit_kernel_prepare+0x44>
	 390:	7c 08 02 a6 	mflr    r0
	 394:	90 01 00 14 	stw     r0,20(r1)
	 398:	48 00 00 01 	bl      398 <interrupt_exit_kernel_prepare+0x50>
				398: R_PPC_REL24	unrecoverable_exception
	 39c:	80 01 00 14 	lwz     r0,20(r1)
	 3a0:	81 3f 00 84 	lwz     r9,132(r31)
	 3a4:	7c 08 03 a6 	mtlr    r0
	 3a8:	4b ff ff b8 	b       360 <interrupt_exit_kernel_prepare+0x18>
	 3ac:	39 02 00 70 	addi    r8,r2,112
	 3b0:	3d 40 00 01 	lis     r10,1
	 3b4:	7c e0 40 28 	lwarx   r7,0,r8
	 3b8:	7c e7 50 78 	andc    r7,r7,r10
	 3bc:	7c e0 41 2d 	stwcx.  r7,0,r8
	 3c0:	40 a2 ff f4 	bne     3b4 <interrupt_exit_kernel_prepare+0x6c>
	 3c4:	38 60 00 01 	li      r3,1
	 3c8:	4b ff ff ac 	b       374 <interrupt_exit_kernel_prepare+0x2c>

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 arch/powerpc/include/asm/interrupt.h | 2 +-
 arch/powerpc/kernel/interrupt.c      | 1 -
 arch/powerpc/kernel/traps.c          | 2 ++
 3 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/interrupt.h b/arch/powerpc/include/asm/interrupt.h
index dcff30e3919b..fa8bfb91f8df 100644
--- a/arch/powerpc/include/asm/interrupt.h
+++ b/arch/powerpc/include/asm/interrupt.h
@@ -411,7 +411,7 @@ DECLARE_INTERRUPT_HANDLER(altivec_assist_exception);
 DECLARE_INTERRUPT_HANDLER(CacheLockingException);
 DECLARE_INTERRUPT_HANDLER(SPEFloatingPointException);
 DECLARE_INTERRUPT_HANDLER(SPEFloatingPointRoundException);
-DECLARE_INTERRUPT_HANDLER(unrecoverable_exception);
+DECLARE_INTERRUPT_HANDLER(unrecoverable_exception) __noreturn;
 DECLARE_INTERRUPT_HANDLER(WatchdogException);
 DECLARE_INTERRUPT_HANDLER(kernel_bad_stack);
 
diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index eca3be36c18c..7e7106641ca9 100644
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c
@@ -440,7 +440,6 @@ notrace unsigned long interrupt_exit_user_prepare(struct pt_regs *regs, unsigned
 	return ret;
 }
 
-void unrecoverable_exception(struct pt_regs *regs);
 void preempt_schedule_irq(void);
 
 notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs, unsigned long msr)
diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index 2afa05ad21c8..1ff776e9e8e3 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -2173,6 +2173,8 @@ DEFINE_INTERRUPT_HANDLER(unrecoverable_exception)
 	pr_emerg("Unrecoverable exception %lx at %lx (msr=%lx)\n",
 		 regs->trap, regs->nip, regs->msr);
 	die("Unrecoverable exception", regs, SIGABRT);
+	/* die() should not return */
+	BUG();
 }
 NOKPROBE_SYMBOL(unrecoverable_exception);
 
-- 
2.25.0