linux-kernel - Re: [PATCH 3/3] jump_label: make initial NOP patching the special case

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Date:   Wed, 15 Jun 2022 10:52:41 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        linux-mips@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH 3/3] jump_label: make initial NOP patching the special
 case

On Wed, Jun 08, 2022 at 12:45:12PM +0200, Ard Biesheuvel wrote:
> Instead of defaulting to patching NOP opcodes at init time, and leaving
> it to the architectures to override this if this is not needed, switch
> to a model where doing nothing is the default. This is the common case
> by far, as only MIPS requires NOP patching at init time. On all other
> architectures, the correct encodings are emitted by the compiler and so
> no initial patching is needed.
> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
>  Documentation/staging/static-keys.rst |  3 ---
>  arch/arc/kernel/jump_label.c          | 13 -------------
>  arch/arm/kernel/jump_label.c          |  6 ------
>  arch/arm64/kernel/jump_label.c        | 11 -----------
>  arch/mips/include/asm/jump_label.h    |  2 ++
>  arch/parisc/kernel/jump_label.c       | 11 -----------
>  arch/riscv/kernel/jump_label.c        | 12 ------------
>  arch/s390/kernel/jump_label.c         |  5 -----
>  arch/x86/kernel/jump_label.c          | 13 -------------
>  kernel/jump_label.c                   | 14 +++-----------
>  10 files changed, 5 insertions(+), 85 deletions(-)

I have one minor comment below, but either way this is a nice cleanup (and I'm
always happy to see __weak functions disappear), so FWIW:

  Acked-by: Mark Rutland <mark.rutland@....com>

[...]

> diff --git a/kernel/jump_label.c b/kernel/jump_label.c
> index b1ac2948be79..ff8576c00893 100644
> --- a/kernel/jump_label.c
> +++ b/kernel/jump_label.c
> @@ -332,17 +332,9 @@ static int __jump_label_text_reserved(struct jump_entry *iter_start,
>  	return 0;
>  }
>  
> -/*
> - * Update code which is definitely not currently executing.
> - * Architectures which need heavyweight synchronization to modify
> - * running code can override this to make the non-live update case
> - * cheaper.
> - */
> -void __weak __init_or_module arch_jump_label_transform_static(struct jump_entry *entry,
> -					    enum jump_label_type type)
> -{
> -	arch_jump_label_transform(entry, type);
> -}
> +#ifndef arch_jump_label_transform_static
> +#define arch_jump_label_transform_static(entry, type)
> +#endif

It might be slightly better to make this a static inline stub so that we always
get the compiler to type-check it, e.g.

| #ifndef arch_jump_label_transform_static
| static inline void arch_jump_label_transform_static(struct jump_entry *entry,
| 						    enum jump_label_type type)
| {
| 	/* nothing to do on most architectures */
| }
| #define arch_jump_label_transform_static arch_jump_label_transform_static
| #endif

Mark.