Re: [PATCH] powerpc/bug: Remove specific powerpc BUG_ON()

[Christophe Leroy]

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@xxxxxx/
> >
> > 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