linux-kernel - Re: [PATCH v4 2/2] powerpc/irq: inline call_do_irq() and call_do

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Date:   Mon, 25 Nov 2019 21:32:23 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v4 2/2] powerpc/irq: inline call_do_irq() and call_do_softirq()

Segher Boessenkool <segher@...nel.crashing.org> writes:
> On Thu, Nov 21, 2019 at 05:14:45PM +1100, Michael Ellerman wrote:
>> Christophe Leroy <christophe.leroy@....fr> writes:
>> That breaks 64-bit with GCC9:
>> 
>>   arch/powerpc/kernel/irq.c: In function 'do_IRQ':
>>   arch/powerpc/kernel/irq.c:650:2: error: PIC register clobbered by 'r2' in 'asm'
>>     650 |  asm volatile(
>>         |  ^~~
>>   arch/powerpc/kernel/irq.c: In function 'do_softirq_own_stack':
>>   arch/powerpc/kernel/irq.c:711:2: error: PIC register clobbered by 'r2' in 'asm'
>>     711 |  asm volatile(
>>         |  ^~~
>> 
>> 
>> > diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
>> > index 04204be49577..d62fe18405a0 100644
>> > --- a/arch/powerpc/kernel/irq.c
>> > +++ b/arch/powerpc/kernel/irq.c
>> > @@ -642,6 +642,22 @@ void __do_irq(struct pt_regs *regs)
>> >  	irq_exit();
>> >  }
>> >  
>> > +static inline void call_do_irq(struct pt_regs *regs, void *sp)
>> > +{
>> > +	register unsigned long r3 asm("r3") = (unsigned long)regs;
>> > +
>> > +	/* Temporarily switch r1 to sp, call __do_irq() then restore r1 */
>> > +	asm volatile(
>> > +		"	"PPC_STLU"	1, %2(%1);\n"
>> > +		"	mr		1, %1;\n"
>> > +		"	bl		%3;\n"
>> > +		"	"PPC_LL"	1, 0(1);\n" :
>> > +		"+r"(r3) :
>> > +		"b"(sp), "i"(THREAD_SIZE - STACK_FRAME_OVERHEAD), "i"(__do_irq) :
>> > +		"lr", "xer", "ctr", "memory", "cr0", "cr1", "cr5", "cr6", "cr7",
>> > +		"r0", "r2", "r4", "r5", "r6", "r7", "r8", "r9", "r10", "r11", "r12");
>> > +}
>> 
>> If we add a nop after the bl, so the linker could insert a TOC restore,
>> then I don't think there's any circumstance under which we expect this
>> to actually clobber r2, is there?
>
> That is mostly correct.

That's the standard I aspire to :P

> If call_do_irq was a no-inline function, there would not be problems.
>
> What TOC does __do_irq require in r2 on entry, and what will be there
> when it returns?

The kernel TOC, and also the kernel TOC, unless something's gone wrong
or I'm missing something.

cheers