Re: [PATCH v4 1/2] arm64: Apply overrides to CPU local capabilities

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

On Tue, Aug 25, 2026 at 05:42:18PM +0100, Fuad Tabba wrote:
> From: Suzuki K Poulose <suzuki.poulose@arm.com>
> 
> If an override has been applied, make sure we apply that for the
> secondary CPUs too, to limit the features.
> 
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> Link: https://lore.kernel.org/all/afc5bd00-28ca-413b-b047-ee53589c285d@arm.com/
> Cc: stable@vger.kernel.org
> [Fuad: whitespace and comment fixes]
> Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>

For completeness, we should add:

Fixes: b3341ae0efa2  ("arm64: cpufeature: Use IDreg override in __read_sysreg_by_encoding()")

> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 9a22df0c5120f..88b15b5ef2f7f 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1232,10 +1232,43 @@ void __init init_cpu_features(struct cpuinfo_arm64 *info)
>  		init_cpu_ftr_reg(SYS_GMID_EL1, info->reg_gmid);
>  }
>  
> +/*
> + * Sanitise the register fields to clamp the values to the overrides that
> + * have been applied.
> + */
> +static u64 override_cpu_ftr_reg(struct arm64_ftr_reg *reg, u64 val)
> +{
> +	const struct arm64_ftr_bits *ftrp;
> +
> +	if (!reg || !reg->override->mask)
> +		return val;
> +
> +	for (ftrp = reg->ftr_bits; ftrp->width; ftrp++) {
> +		u64 ftr_mask = arm64_ftr_mask(ftrp);
> +		s64 ftr_val, ftr_ovr, ftr_safe;
> +
> +		/* Skip the fields not overridden */
> +		if ((ftr_mask & reg->override->mask) != ftr_mask)
> +			continue;
> +
> +		ftr_val = arm64_ftr_value(ftrp, val);
> +		ftr_ovr = arm64_ftr_value(ftrp, reg->override->val);
> +		ftr_safe = arm64_ftr_safe_value(ftrp, ftr_ovr, ftr_val);
> +
> +		if (ftr_safe != ftr_val)
> +			val = arm64_ftr_set_value(ftrp, val, ftr_safe);
> +	}
> +
> +	return val;
> +}
> +
>  static void update_cpu_ftr_reg(struct arm64_ftr_reg *reg, u64 new)
>  {
>  	const struct arm64_ftr_bits *ftrp;
>  
> +	/* Apply the overrides */
> +	new = override_cpu_ftr_reg(reg, new);

Not sure we need this. The init value has already been clamped, made
safe, so it won't change the result.

Otherwise:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>