linux-kernel - Re: [PATCH v3 4/9] arm64: cpufeature: Document the rules of safe value for features

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Date:   Fri, 6 Jan 2017 12:30:11 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        ryan.arnold@...aro.org, sid@...erved-bit.com, aph@...hat.com,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        adhemerval.zanella@...aro.org, dave.martin@....com
Subject: Re: [PATCH v3 4/9] arm64: cpufeature: Document the rules of safe
 value for features

On Wed, Jan 04, 2017 at 05:49:02PM +0000, Suzuki K. Poulose wrote:
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -29,7 +29,21 @@
>  #include <linux/jump_label.h>
>  #include <linux/kernel.h>
>  
> -/* CPU feature register tracking */
> +/*
> + * CPU feature register tracking
> + *
> + * The safe value of a CPUID feature field is dependent on the implications
> + * of the values assigned to it by the architecture. Based on the relationship
> + * between the values, the features are classified into 3 types.
> + *
> + * a) LOWER_SAFE - The value 'n+1' indicates, value 'n' and some
> + *    additional features. (where n >= 0). The smaller value (n) is
> + *    considered safer in this case.
> + * b) HIGHER_SAFE - The value 'n+1' is safer than 'n' (for n>= 0).
> + * c) EXACT - If the values of the feature don't have any relationship,
> + *    a predefined safe value is used.
> + */

I don't think this text fully describes what is actually compared. You
could say something that the lowest value of all the CPUs is chosen for
LOWER_SAFE, highest for HIGHER_SAFE and it is expected that all CPUs
have the same value for a field when EXACT is specified.

-- 
Catalin