[PATCH] powerpc/traps: Declare unrecoverable_exception() as __noreturn
Christophe Leroy
christophe.leroy at csgroup.eu
Thu Feb 11 20:02:58 AEDT 2021
Le 11/02/2021 à 08:47, Gabriel Paubert a écrit :
> On Thu, Feb 11, 2021 at 06:34:43AM +0000, Christophe Leroy wrote:
>> 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,
>
> s/always/allows ?
Yes
>
> (Otherwise I can't parse it.)
>
>> it avoids the stack frame in the likely fast path and avoids code duplication
>> for the exit.
>
> Indeed, nice code generation improvement.
>
>>
>> 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>
>
> Infinite loop (seems to be on test of MSR_PR)?
Yes, that's what you get when CONFIG_BUG is not selected.
/include/asm-generic/bug.h:
#ifndef HAVE_ARCH_BUG
#define BUG() do {} while (1)
#endif
>
> Gabriel
>
>> 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 at 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
>>
>
>
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