Re: [PATCH v4 6/6] arm64: cpufeature: Detect BBML3 based on ID_AA64MMFR2_EL1.BBM

[Will Deacon <will@kernel.org>]

On Thu, Jul 23, 2026 at 10:10:33AM +0530, Linu Cherian wrote:
> Add ID_AA64MMFR2_EL1.BBM based BBML3 feature detection in
> cpu_supports_bbml3() so that cpus with the feature would
> not have to be added into MIDR based supports_bbml3_list.
> 
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
> Signed-off-by: Linu Cherian <linu.cherian@arm.com>
> ---
>  arch/arm64/kernel/cpufeature.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 896bafdb00b1..dbd7d187520c 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -2133,6 +2133,12 @@ static bool hvhe_possible(const struct arm64_cpu_capabilities *entry,
>  
>  bool cpu_supports_bbml3(void)
>  {
> +	u64 mmfr2;
> +
> +	mmfr2 = __read_sysreg_by_encoding(SYS_ID_AA64MMFR2_EL1);
> +	if (SYS_FIELD_GET(ID_AA64MMFR2_EL1, BBM, mmfr2) >= ID_AA64MMFR2_EL1_BBM_3)
> +		return true;

This is a bit of a nit, but I think it would be more consistent to use
has_cpuid_feature() here instead of __read_sysreg_by_encoding(), similarly
to how we handle kpti in unmap_kernel_at_el0() (which also has both an
ID register field and a list of MIDRs).

Will