[RESEND PATCH v6 1/3] arm64: Add BBM Level 2 cpu feature

[Suzuki K Poulose]

On 12/05/2025 14:07, Ryan Roberts wrote:
> On 09/05/2025 17:04, Catalin Marinas wrote:
>> On Fri, May 09, 2025 at 02:49:05PM +0100, Will Deacon wrote:
>>> On Tue, May 06, 2025 at 03:52:59PM +0100, Ryan Roberts wrote:
>>>> On 06/05/2025 15:25, Will Deacon wrote:
>>>>> This penalises large homogeneous systems and it feels unnecessary given
>>>>> that we have the ability to check this per-CPU. Can you use
>>>>> ARM64_CPUCAP_BOOT_CPU_FEATURE instead of ARM64_CPUCAP_SYSTEM_FEATURE
>>>>> to solve this?
>>>>
>>>> We are trying to solve for the case where the boot CPU has BBML2 but a secondary
>>>> CPU doesn't. (e.g. hetrogeneous system where boot CPU is big and secondary is
>>>> little and does not advertise the feature. I can't remember if we proved there
>>>> are real systems with this config - I have vague recollection that we did but my
>>>> memory is poor...).
>>>>
>>>> My understanding is that for ARM64_CPUCAP_BOOT_CPU_FEATURE, "If the boot CPU
>>>> has enabled this feature already, then every late CPU must have it". So that
>>>> would exclude any secondary CPUs without BBML2 from coming online?
>>>
>>> Damn, yes, you're right. However, it still feels horribly hacky to iterate
>>> over the online CPUs in has_bbml2_noabort() -- the cpufeature framework
>>> has the ability to query features locally and we should be able to use
>>> that. We're going to want that should the architecture eventually decide
>>> on something like BBML3 for this.
>>>
>>> What we have with BBML2_NOABORT seems similar to an hwcap in that we only
>>> support the capability if all CPUs have it (rejecting late CPUs without it
>>> in that case) but we can live without it if not all of the early CPUs
>>> have it. Unlikely hwcaps, though, we shouldn't be advertising this to
>>> userspace and we can't derive the capability solely from the sanitised
>>> system registers.
>>>
>>> I wonder if we could treat it like an erratum in some way instead? That
>>> is, invert things so that CPUs which _don't_ have BBML2_NOABORT are
>>> considered to have a "BBM_CONFLICT_ABORT" erratum (which we obviously
>>> wouldn't shout about). Then we should be able to say:
>>>
>>>    - If any of the early CPUs don't have BBML2_NOABORT, then the erratum
>>>      would be enabled and we wouln't elide BBM.
>>>
>>>    - If a late CPU doesn't have BBML2_NOABORT then it can't come online
>>>      if the erratum isn't already enabled.
>>>
>>> Does that work? If not, then perhaps the cpufeature/cpuerrata code needs
>>> some surgery for this.
>>
>> Ah, I should have read this thread in order. I think we can treat this
>> as BBML2_NOABORT available as default based on ID regs and use the
>> allow/deny-list as an erratum.
>>
> 
> Just to make sure I've understood all this, I think what you are both saying is
> we can create a single capability called ARM64_HAS_NO_BBML2_NOABORT of type
> ARM64_CPUCAP_LOCAL_CPU_ERRATUM. Each CPU will then check it has BBML2 and is in
> the MIDR allow list; If any of those conditions are not met, the CPU is
> considered to have ARM64_HAS_NO_BBML2_NOABORT.

I guess we need two caps.

1. SYSTEM cap -> ARM64_HAS_BBML2. Based on the ID registers
2. An erratum -> ARM64_BBML2_ABORTS. Based on BBLM2==1 && !in_midr_list()


And then:


> 
> Then we have this helper:
> 
> static inline bool system_supports_bbml2_noabort(void)
> {
> 	return system_capabilities_finalized() &&
		alternative_has_cap_unlikely(ARM64_HAS_BBML2) &&
		!alternative_has_cap_unlikely(!ARM64_HAS_BBML2_ABORTS)

Without (1), we may enable BBML2 on a (system with) CPU that doesn't
have BBML2 feature.

And (1) can prevent any non-BBML2 capable CPUs from booting or (2) can 
prevent anything that aborts with BBML2.


Suzuki


> 	       !alternative_has_cap_unlikely(ARM64_HAS_NO_BBML2_NOABORT);


> }
> 
> system_capabilities_finalized() is there to ensure an early call to this helper
> returns false (i.e. the safe value before we have evaluated on all CPUs).
> Because ARM64_HAS_NO_BBML2_NOABORT is inverted it would otherwise return true
> prior to finalization.
> 
> I don't believe we need any second (SYSTEM or BOOT) feature. This is sufficient
> on its own?
> 
> Thanks,
> Ryan
>