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

On 03/08/26 9:22 AM, Linu Cherian wrote:
> 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 <[email protected]>
>>> Reviewed-by: Anshuman Khandual <[email protected]>
>>> Signed-off-by: Linu Cherian <[email protected]>
>>> ---
>>>  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 ?
Right - that would be useful here while also providing some
context why has_cpuid_feature() could not have been used.