linux-kernel - Re: [PATCH v2 06/18] arm64: arch_timer: Add infrastructure for multiple erratum detection methods

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Message-ID: <36733343-8f16-f009-dd4d-1ee63614b053@arm.com>
Date:   Wed, 29 Mar 2017 15:56:52 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Scott Wood <oss@...error.net>,
        Hanjun Guo <hanjun.guo@...aro.org>,
        Ding Tianhong <dingtianhong@...wei.com>,
        dann frazier <dann.frazier@...onical.com>
Subject: Re: [PATCH v2 06/18] arm64: arch_timer: Add infrastructure for
 multiple erratum detection methods

On 29/03/17 15:27, Daniel Lezcano wrote:
> On Tue, Mar 28, 2017 at 04:38:41PM +0100, Marc Zyngier wrote:
>> On 28/03/17 15:55, Daniel Lezcano wrote:
>>> On Tue, Mar 28, 2017 at 03:48:23PM +0100, Marc Zyngier wrote:
>>>> On 28/03/17 15:36, Daniel Lezcano wrote:
>>>>> On Tue, Mar 28, 2017 at 03:07:52PM +0100, Marc Zyngier wrote:
>>>>>
>>>>> [ ... ]
>>>>>
>>>>>>>>> -bool arch_timer_check_global_cap_erratum(const struct arch_timer_erratum_workaround *wa,
>>>>>>>>> -					 const void *arg)
>>>>>>>>> +bool arch_timer_check_cap_erratum(const struct arch_timer_erratum_workaround *wa,
>>>>>>>>> +				  const void *arg)
>>>>>>>>>  {
>>>>>>>>> -	return cpus_have_cap((uintptr_t)wa->id);
>>>>>>>>> +	return cpus_have_cap((uintptr_t)wa->id) | this_cpu_has_cap((uintptr_t)wa->id);
>>>>>>>>
>>>>>>>> Not quite. Here, you're making all capability-based errata to be be
>>>>>>>> global (if a single CPU in the system has a capability, then by
>>>>>>>> transitivity cpus_have_cap returns true). If that's a big-little system,
>>>>>>>> you end-up applying the workaround to all CPUs, including those unaffected.
>>>>>>>>
>>>>>>>> I'd rather drop cpus_have_cap altogether and rely on individual CPU
>>>>>>>> matching (since we don't have a need for a global capability erratum
>>>>>>>> handling yet).
>>>>>>>
>>>>>>> Ok, thanks.
>>>>>>
>>>>>> Quick update. I've just implemented this, and found out that getting rid
>>>>>> of local/global has an unfortunate effect:
>>>>>>
>>>>>> Since we only probe the global errata (using ACPI for example) on the
>>>>>> boot CPU path, we lose propagation of the erratum across the secondary
>>>>>> CPUs. One way of solving this is to convert the secondary boot path to
>>>>>> be aware of DT vs ACPI vs detection method of the month. Which isn't
>>>>>> easy, since by the time we boot secondary CPUs, we don't have the
>>>>>> pointers to the various ACPI tables anymore. Also, assuming we were
>>>>>> careful and saved the pointers, the tables may have been unmapped. Fun.
>>>>>
>>>>> My proposal was supposed to prevent that. The detecion is done in the
>>>>> subsystems, ACPI detects ACPI errata, DT detects DT errata and CPU detects CPU
>>>>> errata. The drivers get the errata and enable the workaround. The id
>>>>> association <-> errata self contains errata types (void *, char *, int). So
>>>>> everything can be done in a CPU basis without local / global dance.
>>>>
>>>> I'm sorry, but it feels like a Jumbo-Jet sized hammer to try and squash
>>>> a fly (I'm staying away from the frozen shark metaphor here). You're
>>>> willing to add a whole list of things with private ids that need
>>>> matching to kill a flag? I don't think this buys us anything but extra
>>>> complexity and another maintenance headache.
>>>
>>> Well, it is like your approach except it is split in two steps.
>>>
>>> Can you explain where is the extra complexity ? May be I am missing the point.
>>
>> This is how I understand your approach:
>>
>> - Boot the first CPU
>> - Build a list of errata discovered at that time
>> - Apply erratum on the boot CPU if required, using a yet-to-be-invented
>> private id matching mechanism,
>> - Boot a secondary CPU
>> - Apply erratum if required, parsing the list
>> - Realise that you don't have the full list (this CPU comes with an
>> erratum that was not in the initial list)
>> - Add more to the list
>> - Apply erratum, using the same matching mechanism
>>
>> This is mine:
>>
>> - Boot the first CPU
>> - Apply global erratum to all CPUs
>> - Apply local erratum
>> - Boot a secondary CPU
>> - Apply local erratum
>>
>> In my case, everything is static, and I don't need to rematch each CPU
>> against the list of globally applicable errata.
>>
>> If my understanding is flawed, let me know.
> 
> Any of our understanding is flawed. I think that needs a maturation period.

Well, these patches have been maturing for a while, and time is running
out. If you have a better idea that is more than a concept, please post
the code, I'd be happy to review it.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...