linux-kernel - Re: [PATCH 05/16] arm64: Add flags to check the safety of a capability for late CPU

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Date:   Fri, 26 Jan 2018 10:10:21 +0000
From:   Dave Martin <Dave.Martin@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        ckadabi@...eaurora.org, ard.biesheuvel@...aro.org,
        marc.zyngier@....com, catalin.marinas@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, jnair@...iumnetworks.com
Subject: Re: [PATCH 05/16] arm64: Add flags to check the safety of a
 capability for late CPU

On Tue, Jan 23, 2018 at 12:27:58PM +0000, Suzuki K Poulose wrote:
> Add two different flags to indicate if the conflict of a capability
> on a late CPU with the current system state
> 
>  1) Can a CPU have a capability when the system doesn't have it ?
> 
>     Most arm64 features could have this set. While erratum work arounds
>     cannot have this, as we may miss work arounds.
> 
>  2) Can a CPU miss a capability when the system has it ?
>     This could be set for arm64 erratum work arounds as we don't
>     care if a CPU doesn't need the work around. However it should
>     be clear for features.
> 
> These flags could be added to certain entries based on their nature.
> 
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> ---
>  arch/arm64/include/asm/cpufeature.h | 35 +++++++++++++++++++++++++++++++----
>  1 file changed, 31 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 4fd5de8ef33e..27d037bb0451 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -94,10 +94,25 @@ extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0;
>  #define SCOPE_SYSTEM				ARM64_CPUCAP_SCOPE_SYSTEM
>  #define SCOPE_LOCAL_CPU				ARM64_CPUCAP_SCOPE_LOCAL_CPU
>  
> -/* CPU errata detected at boot time based on feature of one or more CPUs */
> -#define ARM64_CPUCAP_STRICT_CPU_LOCAL_ERRATUM	(ARM64_CPUCAP_SCOPE_LOCAL_CPU)
> -/* CPU feature detected at boot time based on system-wide value of a feature */
> -#define ARM64_CPUCAP_BOOT_SYSTEM_FEATURE	(ARM64_CPUCAP_SCOPE_SYSTEM)
> +/* Is it safe for a late CPU to have this capability when system doesn't already have */
> +#define ARM64_CPUCAP_LATE_CPU_SAFE_TO_HAVE	BIT(2)
> +/* Is it safe for a late CPU to miss this capability when system has it */
> +#define ARM64_CPUCAP_LATE_CPU_SAFE_TO_MISS	BIT(3)

Maybe _OPTIONAL and _PERMITTED would be a bit less verbose?

Alternatively,
	ARM64_CPUCAP_PERMITTED_FOR_LATE_CPU
	ARM64_CPUCAP_OPTIONAL_FOR_LATE_CPU

might be easier to understand.

[...]

Cheers
---Dave