Re: [PATCH] arm64: rsi: Correct type of realm_config::hash_algo

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Sep 29, 2026 at 08:36:00PM +0900, Kohei Enju wrote:
> The RMM specification defines RsiHashAlgorithm as an 8-bit type, but
> realm_config::hash_algo is currently declared as unsigned long.
> 
> There are currently no upstream users of this field, so the mismatch
> does not cause an observable issue today. However, a proposed series
> adding support for Realm measurement registers uses this field and
> exposes a bug caused by the incorrect type.
> 
> Change hash_algo to u8 to match the RMM specification while preserving
> the size and layout of struct realm_config.

Doesn't the RMM spec say that the unused bits must be zero? B5.5.19.
That said, I'm ok with the explicit u8 here.

> Link: https://lore.kernel.org/all/20260413084957.327661-4-sami.mujawar@arm.com/
> Signed-off-by: Kohei Enju <enju.kohei@fujitsu.com>
> ---
>  include/linux/arm-smccc-rsi.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/arm-smccc-rsi.h b/include/linux/arm-smccc-rsi.h
> index fddb77986f70..2fb6ec5e0298 100644
> --- a/include/linux/arm-smccc-rsi.h
> +++ b/include/linux/arm-smccc-rsi.h
> @@ -128,7 +128,10 @@ struct realm_config {
>  	union {
>  		struct {
>  			unsigned long ipa_bits; /* Width of IPA in bits */
> -			unsigned long hash_algo; /* Hash algorithm */
> +			union {
> +				u8 hash_algo; /* Hash algorithm */
> +				u8 pad_hash_algo[8];
> +			};

I would skip the padding for now, we have one already. If we later add
the next fields, I have a slight preference for a u8 reserved[7] field
than another union.

>  		};
>  		u8 pad[0x200];
>  	};

-- 
Catalin