linux-kernel - Re: [PATCH v2 9/9] arm64: Work around systems with mismatched cache line sizes

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Message-ID: <f72be21a-fd67-dc30-ca1b-278dd99821a2@arm.com>
Date:   Fri, 26 Aug 2016 14:08:01 +0100
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Mark Rutland <mark.rutland@....com>,
        Andre Przywara <andre.przywara@....com>
Subject: Re: [PATCH v2 9/9] arm64: Work around systems with mismatched cache
 line sizes

On 26/08/16 14:04, Suzuki K Poulose wrote:
> On 26/08/16 12:03, Ard Biesheuvel wrote:
>> Hello Suzuki,
>>
>
>
>>> For faster access (i.e, avoiding to lookup the system wide value of CTR_EL0
>>> via read_system_reg), we keep track of the pointer to table entry for
>>> CTR_EL0 in the CPU feature infrastructure.
>>>
>>
>> IIUC it is the runtime sorting of the arm64_ftr_reg array that
>> requires you to stash a pointer to CTR_EL0's entry somewhere, so that
>> you can dereference it without doing the bsearch.
>
> Correct.



>
>>
>> IMO, this is a pattern that we should avoid: you are introducing one
>> instance now, which will make it hard to say no to the next one in the
>> future. Isn't there a better way to organize the arm64_ftr_reg array
>> that allows us to reference entries directly? Ideally, a way that gets
>> rid of the runtime sorting, since I don't think that is a good
>> replacement for developer discipline anyway (although I should have
>> spoken up when that was first introduced) Or am I missing something
>> here?
>
> I had some form of direct access to the feature register in one of
> the versions [0], but was dropped based on Catalin's suggestion at [1].

Forgot to add, [0] wouldn't solve this issue cleanly either. It would simply
speed up the read_system_reg(). So we do need a call to read_system_reg()
from assembly code, which makes it a little bit tricky.

Suzuki

>
>
> [0] https://lkml.org/lkml/2015/10/5/504
> [1] https://lkml.org/lkml/2015/10/7/558
>
>
> Suzuki
>
>