linux-kernel - Re: [PATCH] powerpc/bug: Remove specific powerpc BUG

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Date:   Thu, 11 Feb 2021 16:30:51 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, npiggin@...il.com,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/bug: Remove specific powerpc BUG_ON()



Le 11/02/2021 à 15:30, Segher Boessenkool a écrit :
> On Thu, Feb 11, 2021 at 03:09:43PM +0100, Christophe Leroy wrote:
>> Le 11/02/2021 à 12:49, Segher Boessenkool a écrit :
>>> On Thu, Feb 11, 2021 at 07:41:52AM +0000, Christophe Leroy wrote:
>>>> powerpc BUG_ON() is based on using twnei or tdnei instruction,
>>>> which obliges gcc to format the condition into a 0 or 1 value
>>>> in a register.
>>>
>>> Huh?  Why is that?
>>>
>>> Will it work better if this used __builtin_trap?  Or does the kernel only
>>> detect very specific forms of trap instructions?
>>
>> We already made a try with __builtin_trap() 1,5 year ago, see
>> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20510ce03cc9463f1c9e743c1d93b939de501b53.1566219503.git.christophe.leroy@c-s.fr/
>>
>> The main problems encountered are:
>> - It is only possible to use it for BUG_ON, not for WARN_ON because GCC
>> considers it as noreturn. Is there any workaround ?
> 
> A trap is noreturn by definition:
> 
>   -- Built-in Function: void __builtin_trap (void)
>       This function causes the program to exit abnormally.
> 
>> - The kernel (With CONFIG_DEBUG_BUGVERBOSE) needs to be able to identify
>> the source file and line corresponding to the trap. How can that be done
>> with __builtin_trap() ?
> 
> The DWARF debug info should be sufficient.  Perhaps you can post-process
> some way?
> 
> You can create a trap that falls through yourself (by having a trap-on
> condition with a condition that is always true, but make the compiler
> not see that).  This isn't efficient though.
> 
> Could you file a feature request (in bugzilla)?  It is probably useful
> for generic code as well, but we could implement this for powerpc only
> if needed.
> 

Ok I will.


For sure using __builtin_trap() would be the best.

unsigned long test1(unsigned long msr)
{
	WARN_ON((msr & (MSR_DR | MSR_IR | MSR_RI)) != (MSR_DR | MSR_IR | MSR_RI));

	return msr;
}

unsigned long test2(unsigned long msr)
{
	if ((msr & (MSR_DR | MSR_IR | MSR_RI)) != (MSR_DR | MSR_IR | MSR_RI))
		__builtin_trap();

	return msr;
}

000003c0 <test1>:
  3c0:	70 69 00 32 	andi.   r9,r3,50
  3c4:	69 29 00 32 	xori    r9,r9,50
  3c8:	31 49 ff ff 	addic   r10,r9,-1
  3cc:	7d 2a 49 10 	subfe   r9,r10,r9
  3d0:	0f 09 00 00 	twnei   r9,0
  3d4:	4e 80 00 20 	blr

000003d8 <test2>:
  3d8:	70 69 00 32 	andi.   r9,r3,50
  3dc:	0f 09 00 32 	twnei   r9,50
  3e0:	4e 80 00 20 	blr

Christophe