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

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Date:   Thu, 11 Feb 2021 15:09:43 +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 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 ?
- 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() ?

Christophe