linux-kernel - Re: [PATCH v3 1/9] arm64: cpufeature: treat unknown fields as RES0

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Message-ID: <20170105170826.GC29765@e104818-lin.cambridge.arm.com>
Date:   Thu, 5 Jan 2017 17:08:27 +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 1/9] arm64: cpufeature: treat unknown fields as RES0

On Wed, Jan 04, 2017 at 05:48:59PM +0000, Suzuki K. Poulose wrote:
> From: Mark Rutland <mark.rutland@....com>
> 
> Any fields not defined in an arm64_ftr_bits entry are propagated to the
> system-wide register value in init_cpu_ftr_reg(), and while we require
> that these strictly match for the sanity checks, we don't update them in
> update_cpu_ftr_reg().
> 
> Generally, the lack of an arm64_ftr_bits entry indicates that the bits
> are currently RES0 (as is the case for the upper 32 bits of all
> supposedly 32-bit registers).
> 
> A better default would be to use zero for the system-wide value of
> unallocated bits, making all register checking consistent, and allowing
> for subsequent simplifications to the arm64_ftr_bits arrays.
> 
> This patch updates init_cpu_ftr_reg() to treat unallocated bits as RES0
> for the purpose of the system-wide safe value. These bits will still be
> sanity checked with strict match requirements, as is currently the case.
> 
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will.deacon@....com>
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>

Reviewed-by: Catalin Marinas <catalin.marinas@....com>