linux-kernel - Re: [PATCH v11 1/4] arm64: cpufeature: introduce helper cpu_has_hw

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Date:   Thu, 10 Oct 2019 17:43:12 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Jia He <justin.he@....com>
Cc:     Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Marc Zyngier <maz@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Suzuki Poulose <Suzuki.Poulose@....com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>, hejianet@...il.com,
        Kaly Xin <Kaly.Xin@....com>, nd@....com
Subject: Re: [PATCH v11 1/4] arm64: cpufeature: introduce helper
 cpu_has_hw_af()

On Wed, Oct 09, 2019 at 04:42:43PM +0800, Jia He wrote:
> We unconditionally set the HW_AFDBM capability and only enable it on
> CPUs which really have the feature. But sometimes we need to know
> whether this cpu has the capability of HW AF. So decouple AF from
> DBM by a new helper cpu_has_hw_af().
> 
> Signed-off-by: Jia He <justin.he@....com>
> Suggested-by: Suzuki Poulose <Suzuki.Poulose@....com>
> Reviewed-by: Catalin Marinas <catalin.marinas@....com>

I don't think I reviewed this version of the patch.

> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 9cde5d2e768f..1a95396ea5c8 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -659,6 +659,20 @@ static inline u32 id_aa64mmfr0_parange_to_phys_shift(int parange)
>  	default: return CONFIG_ARM64_PA_BITS;
>  	}
>  }
> +
> +/* Check whether hardware update of the Access flag is supported */
> +static inline bool cpu_has_hw_af(void)
> +{
> +	if (IS_ENABLED(CONFIG_ARM64_HW_AFDBM)) {

Please just return early here to avoid unnecessary indentation:

	if (!IS_ENABLED(CONFIG_ARM64_HW_AFDBM))
		return false;

> +		u64 mmfr1 = read_cpuid(ID_AA64MMFR1_EL1);
> +
> +		return !!cpuid_feature_extract_unsigned_field(mmfr1,
> +						ID_AA64MMFR1_HADBS_SHIFT);

No need for !!, the return type is a bool already.

Anyway, apart from these nitpicks, the patch is fine you can keep my
reviewed-by.

If later we noticed a potential performance issue on this path, we can
turn it into a static label as with other CPU features.

-- 
Catalin