[PATCH v3 02/13] arm64: cpufeatures: Correctly handle signed values

[Will Deacon]

On Mon, Nov 27, 2023 at 11:45:48AM +0000, Marc Zyngier wrote:
> Although we've had signed values for some features such as PMUv3
> and FP, the code that handles the comparaison with some limit
> has a couple of annoying issues:
> 
> - the min_field_value is always unsigned, meaning that we cannot
>   easily compare it with a negative value
> 
> - it is not possible to have a range of values, let alone a range
>   of negative values
> 
> Fix this by:
> 
> - adding an upper limit to the comparison, defaulting to all bits
>   being set to the maximum positive value
> 
> - ensuring that the signess of the min and max values are taken into
>   account
> 
> A ARM64_CPUID_FIELDS_NEG() macro is provided for signed features, but
> nothing is using it yet.
> 
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> ---
>  arch/arm64/include/asm/cpufeature.h |  1 +
>  arch/arm64/kernel/cpufeature.c      | 65 +++++++++++++++++++++++++----
>  2 files changed, 57 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index f6d416fe49b0..5f3f62efebd5 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -363,6 +363,7 @@ struct arm64_cpu_capabilities {
>  			u8 field_pos;
>  			u8 field_width;
>  			u8 min_field_value;
> +			u8 max_field_value;
>  			u8 hwcap_type;
>  			bool sign;
>  			unsigned long hwcap;
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 646591c67e7a..bc8787f28ffd 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -140,12 +140,42 @@ void dump_cpu_features(void)
>  	pr_emerg("0x%*pb\n", ARM64_NCAPS, &system_cpucaps);
>  }
>  
> +#define __ARM64_MAX_POSITIVE(reg, field)				\
> +		((reg##_##field##_SIGNED ?				\
> +		  BIT(reg##_##field##_WIDTH - 1) :			\
> +		  BIT(reg##_##field##_WIDTH)) - 1)
> +
> +#define __ARM64_MIN_NEGATIVE(reg, field)  BIT(reg##_##field##_WIDTH - 1)

I'm struggling to grok these two macros. For example, let's say I have a
4-bit signed field. In that case, the maximum positive value is 7 (0b0111)
and the minimum negative value is -8 (0b1000), but the macros above appear
to give 0b1000 for both.

Will