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

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Date:   Fri, 26 Aug 2016 17:58:08 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Suzuki K Poulose <Suzuki.Poulose@....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 August 2016 at 17:16, Will Deacon <will.deacon@....com> wrote:
> On Fri, Aug 26, 2016 at 02:08:01PM +0100, Suzuki K Poulose wrote:
>> On 26/08/16 14:04, Suzuki K Poulose wrote:
>> >On 26/08/16 12:03, Ard Biesheuvel wrote:
>> >>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.
>
> It might be worth looking to see if we can pass the ctr as an extra
> parameter to the assembly routines that need it. Then you can access it
> easily from C code, and if you pass it as 0 that could result in the asm
> code reading it from the h/w register, removing the need for the _raw
> stuff you add.
>
> Of course, it could also be a complete mess fixing up all the callers,
> but it's probably worth investigating to see what the trade-off is.
>

The point Catalin made was under the assumption that this code is
never called on a hot path, and now you are finding yourself
optimizing the access, which either means you are doing so
prematurely, or it is on a hot path.

I will follow up with a separate suggestion. Feel free to incorporate
it, or completely ignore it. Just my 2 cents.

-- 
Ard.