linux-kernel - Re: [PATCH v3 9/9] arm64: Work around systems with mismatched cache line sizes

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Date:   Mon, 5 Sep 2016 11:24:02 +0100
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Andre Przywara <andre.przywara@....com>
Subject: Re: [PATCH v3 9/9] arm64: Work around systems with mismatched cache
 line sizes

On 05/09/16 11:10, Ard Biesheuvel wrote:
> On 5 September 2016 at 10:58, Suzuki K Poulose <suzuki.poulose@....com> wrote:
>> +/*
>> + * read_ctr - read CTR_EL0. If the system has mismatched
>> + * cache line sizes, provide the system wide safe value.
>> + */
>> +       .macro  read_ctr, reg
>> +alternative_if_not ARM64_MISMATCHED_CACHE_LINE_SIZE
>> +       mrs     \reg, ctr_el0                   // read CTR
>> +       nop
>> +       nop
>> +alternative_else
>> +       adr_l   \reg, arm64_ftr_reg_ctrel0      // Read system wide safe CTR value
>> +       ldr     \reg, [\reg, #ARM64_FTR_SYSVAL] // from arm64_ftr_reg_ctrel0.sys_val
>
> You should be able to use
>
> ldr_l \reg, arm64_ftr_reg_ctrel0 + ARM64_FTR_SYSVAL
>
> here, and save one instruction.

I had given a thought about that and chose the above to account for a
rare chance of arm64_ftr_reg_ctrel0 spanning across a 4K boundary. But,
you are right, ldr_l could treat (arm64_ftr_reg_ctrel0 + ARM64_FTR_SYSVAL)
as the symbol address and still get the right offset.

Suzuki