Subject: Re: [PATCH v4 11/20] powerpc: Add ZEROIZE_GPRS macros for register clears

From: "Nicholas Piggin" <[email-masked]>
Date: Mon, 12 Sep 2022 21:09:36 +1000

On Wed Aug 24, 2022 at 12:05 PM AEST, Rohan McLure wrote:
> Macros for restoring and saving registers to and from the stack exist.
> Provide macros with the same interface for clearing a range of gprs by
> setting each register's value in that range to zero.

Can I bikeshed this a bit more and ask if you would change the order?

Saving and restoring macros are incidental, and are neither the change
nor the reson for it. I think the changelog reads better if you state
the change (or the problem) up front.

Provid register zeroing macros ... follow the same convention as
existing register stack save/restore macros ... will be used in later
change to zero registers.

Or not, if you disagree.

Reviewed-by: Nicholas Piggin <[email-masked]>

> The resulting macros are called ZEROIZE_GPRS and ZEROIZE_NVGPRS, keeping
> with the naming of the accompanying restore and save macros, and usage
> of zeroize to describe this operation elsewhere in the kernel.
>
> Signed-off-by: Rohan McLure <[email-masked]>
> ---
> V1 -> V2: Change 'ZERO' usage in naming to 'NULLIFY', a more obvious verb
> V2 -> V3: Change 'NULLIFY' usage in naming to 'ZEROIZE', which has
> precedent in kernel and explicitly specifies that we are zeroing.
> V3 -> V4: Update commit message to use zeroize.
> ---
>  arch/powerpc/include/asm/ppc_asm.h | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/arch/powerpc/include/asm/ppc_asm.h b/arch/powerpc/include/asm/ppc_asm.h
> index 83c02f5a7f2a..b95689ada59c 100644
> --- a/arch/powerpc/include/asm/ppc_asm.h
> +++ b/arch/powerpc/include/asm/ppc_asm.h
> @@ -33,6 +33,20 @@
>  	.endr
>  .endm
>  
> +/*
> + * This expands to a sequence of register clears for regs start to end
> + * inclusive, of the form:
> + *
> + *   li rN, 0
> + */
> +.macro ZEROIZE_REGS start, end
> +	.Lreg=\start
> +	.rept (\end - \start + 1)
> +	li	.Lreg, 0
> +	.Lreg=.Lreg+1
> +	.endr
> +.endm
> +
>  /*
>   * Macros for storing registers into and loading registers from
>   * exception frames.
> @@ -49,6 +63,14 @@
>  #define REST_NVGPRS(base)		REST_GPRS(13, 31, base)
>  #endif
>  
> +#define	ZEROIZE_GPRS(start, end)	ZEROIZE_REGS start, end
> +#ifdef __powerpc64__
> +#define	ZEROIZE_NVGPRS()		ZEROIZE_GPRS(14, 31)
> +#else
> +#define	ZEROIZE_NVGPRS()		ZEROIZE_GPRS(13, 31)
> +#endif
> +#define	ZEROIZE_GPR(n)			ZEROIZE_GPRS(n, n)
> +
>  #define SAVE_GPR(n, base)		SAVE_GPRS(n, n, base)
>  #define REST_GPR(n, base)		REST_GPRS(n, n, base)
>  
> -- 
> 2.34.1