Re: [PATCH 2/4] powerpc/kuap: Avoid useless jump_label on empty function

[Nicholas Piggin]

On Mon Jun 5, 2023 at 9:04 PM AEST, Christophe Leroy wrote:
> Disassembly of interrupt_enter_prepare() shows a pointless nop before the mftb
>
>   c000abf0 <interrupt_enter_prepare>:
>   c000abf0:       81 23 00 84     lwz     r9,132(r3)
>   c000abf4:       71 29 40 00     andi.   r9,r9,16384
>   c000abf8:       41 82 00 28     beq-    c000ac20 <interrupt_enter_prepare+0x30>
>   c000abfc: ===>  60 00 00 00     nop	<====
>   c000ac00:       7d 0c 42 e6     mftb    r8
>   c000ac04:       80 e2 00 08     lwz     r7,8(r2)
>   c000ac08:       81 22 00 28     lwz     r9,40(r2)
>   c000ac0c:       91 02 00 24     stw     r8,36(r2)
>   c000ac10:       7d 29 38 50     subf    r9,r9,r7
>   c000ac14:       7d 29 42 14     add     r9,r9,r8
>   c000ac18:       91 22 00 08     stw     r9,8(r2)
>   c000ac1c:       4e 80 00 20     blr
>   c000ac20:       60 00 00 00     nop
>   c000ac24:       7d 5a c2 a6     mfmd_ap r10
>   c000ac28:       3d 20 de 00     lis     r9,-8704
>   c000ac2c:       91 43 00 b0     stw     r10,176(r3)
>   c000ac30:       7d 3a c3 a6     mtspr   794,r9
>   c000ac34:       4e 80 00 20     blr
>
> That comes from the call to kuap_loc(), allthough __kuap_lock() is an empty
> function on the 8xx.
>
> To avoid that, only perform kuap_is_disabled() check when there is something
> to do with __kuap_lock().
>
> Do the same with __kuap_save_and_lock() and __kuap_get_and_assert_locked().

Too bad static branch nops can't be eliminated.

> diff --git a/arch/powerpc/include/asm/book3s/64/kup.h b/arch/powerpc/include/asm/book3s/64/kup.h
> index 54cf46808157..1b0215ff3710 100644
> --- a/arch/powerpc/include/asm/book3s/64/kup.h
> +++ b/arch/powerpc/include/asm/book3s/64/kup.h
> @@ -297,15 +297,7 @@ static inline unsigned long __kuap_get_and_assert_locked(void)
>  		WARN_ON_ONCE(amr != AMR_KUAP_BLOCKED);
>  	return amr;
>  }
> -
> -/* Do nothing, book3s/64 does that in ASM */
> -static inline void __kuap_lock(void)
> -{
> -}
> -
> -static inline void __kuap_save_and_lock(struct pt_regs *regs)
> -{
> -}
> +#define __kuap_get_and_assert_locked __kuap_get_and_assert_locked

Maybe leave in /* __kuap_lock notrequired, book3s/64 does that in ASM */
? Seems okay though

Reviewed-by: Nicholas Piggin <npiggin@xxxxxxxxx>

Thanks,
Nick