Re: [PATCH] powerpc/irq: Fix missing r2 clobber in PCREL inline assembly

[Segher Boessenkool <segher@kernel.crashing.org>]

Hi!

On Mon, Jul 20, 2026 at 12:43:47PM +0530, Saket Kumar Bhaskar wrote:
> In CONFIG_PPC_KERNEL_PCREL mode, r2 is no longer reserved for the TOC
> pointer and is available as a caller-saved register [0].

Just like on many more ABIs.  Yeah.  It's the first that PowerPC Linux
supports though :-)

> Both call_do_irq() and call_do_softirq() use inline assembly to call
> functions with stack switching, but fail to list r2 in their clobber
> lists. This causes the compiler to assume r2 is preserved across these
> calls, leading to register corruption when the called functions
> (__do_irq and __do_softirq) clobber r2.

Yeah.

> index a0e8b998c9b5..26df38bdd334 100644
> --- a/arch/powerpc/kernel/irq.c
> +++ b/arch/powerpc/kernel/irq.c
> @@ -218,7 +218,12 @@ static __always_inline void call_do_softirq(const void *sp)
>  		   [callee] "i" (__do_softirq)
>  		 : // Clobbers
>  		   "lr", "xer", "ctr", "memory", "cr0", "cr1", "cr5", "cr6",
> -		   "cr7", "r0", "r3", "r4", "r5", "r6", "r7", "r8", "r9", "r10",
> +		   "cr7", "r0",
> +		  /* r2 is clobbered in PCREL mode */

"Note that r2 may be clobbered in the call, when using the ELF V2 ABI"?

PCREL has nothing to do with it.  Well, not directly :-)  And the
clobber list does not specify registers that definitely *are* clobbered,
but registers that *may be* clobbered, instead :-)  Registers that *are*
clobbered are in the output list (and given a name there so that the new
value can be accessed, etc.)

> +#ifdef CONFIG_PPC_KERNEL_PCREL
> +		   "r2",
> +#endif
> +		   "r3", "r4", "r5", "r6", "r7", "r8", "r9", "r10",
>  		   "r11", "r12"
>  	);
>  }
> @@ -276,7 +281,12 @@ static __always_inline void call_do_irq(struct pt_regs *regs, void *sp)
>  		   [callee] "i" (__do_irq)
>  		 : // Clobbers
>  		   "lr", "xer", "ctr", "memory", "cr0", "cr1", "cr5", "cr6",
> -		   "cr7", "r0", "r4", "r5", "r6", "r7", "r8", "r9", "r10",
> +		   "cr7", "r0",
> +		  /* r2 is clobbered in PCREL mode */
> +#ifdef CONFIG_PPC_KERNEL_PCREL
> +		   "r2",
> +#endif
> +		   "r4", "r5", "r6", "r7", "r8", "r9", "r10",
>  		   "r11", "r12"
>  	);
>  }


Segher