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

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Date:   Tue, 30 Jan 2018 11:17:38 +0000
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Dave Martin <Dave.Martin@....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 26/01/18 10:10, Dave Martin wrote:
> 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

Sounds better than what I have. I have picked them up.

Cheers
Suzuki