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

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Message-ID: <015746d7-ca46-4978-a441-09fba781fdd4@arm.com>
Date: Mon, 12 May 2025 14:07:39 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>
Cc: Mikołaj Lenczewski <miko.lenczewski@....com>,
 suzuki.poulose@....com, yang@...amperecomputing.com, corbet@....net,
 jean-philippe@...aro.org, robin.murphy@....com, joro@...tes.org,
 akpm@...ux-foundation.org, paulmck@...nel.org, mark.rutland@....com,
 joey.gouly@....com, maz@...nel.org, james.morse@....com, broonie@...nel.org,
 oliver.upton@...ux.dev, baohua@...nel.org, david@...hat.com,
 ioworker0@...il.com, jgg@...pe.ca, nicolinc@...dia.com, mshavit@...gle.com,
 jsnitsel@...hat.com, smostafa@...gle.com, kevin.tian@...el.com,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev
Subject: Re: [RESEND PATCH v6 1/3] arm64: Add BBM Level 2 cpu feature

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.

Then we have this helper:

static inline bool system_supports_bbml2_noabort(void)
{
	return system_capabilities_finalized() &&
	       !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