Re: [PATCH v4 2/2] powerpc/irq: inline call_do_irq() and call_do_softirq()

Segher Boessenkool
2019-11-21T10:16:33Z

On Thu, Nov 21, 2019 at 05:14:45PM +1100, Michael Ellerman wrote:
> Christophe Leroy <christophe.leroy@c-s.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.

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?

Segher