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

[Linu Cherian <linu.cherian@arm.com>]

Hi Will,

On Fri, Jul 31, 2026 at 04:13:20PM +0100, Will Deacon wrote:
> 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).

force_pte_mapping required by map_mem(during early boot) needs
cpu_supports_bbml3 check and cpu features/capabilities are not 
initialized by that time. Should i add a comment there to clarify this ?

Thanks,
Linu Cherian.