Re: [PATCH v3 13/20] arm64: cpufeature: Enable IESB on exception entry/return for firmware-first

[James Morse <james.morse@arm.com>]

Hi Catalin,

On 18/10/17 17:43, Catalin Marinas wrote:
> On Thu, Oct 05, 2017 at 08:18:05PM +0100, James Morse wrote:
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index b68f5e93baac..29df2a93688c 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -989,6 +989,21 @@ config ARM64_RAS_EXTN
>>  	  and access the new registers if the system supports the extension.
>>  	  Platform RAS features may additionally depend on firmware support.
>>  
>> +config ARM64_IESB
>> +	bool "Enable Implicit Error Synchronization Barrier (IESB)"
>> +	default y
>> +	depends on ARM64_RAS_EXTN
>> +	help
>> +	  ARM v8.2 adds a feature to add implicit error synchronization
>> +	  barriers whenever the CPU enters or exits a particular exception
>> +	  level.
>> +
>> +	  On CPUs with this feature and the 'RAS Extensions' feature, we can
>> +	  use this to contain detected (but not yet reported) errors to the
>> +	  relevant exception level.
>> +
>> +	  The feature is detected at runtime, selecting this option will
>> +	  enable these implicit barriers if the CPU supports the feature.
>>  endmenu
> 
> What's the use-case for not having this option always enabled?

I don't think there is a strong reason. It ended up with a Kconfig entry just
because its a separate cpufeature entry. I will merge it with the ARM64_RAS_EXTN.

The only reason I can think to turn it off is if its implemented but expensive
on some system, and the EL3/Secure RAS firmware policy stuff doesn't care
whether RAS errors cross exception boundaries.


Thanks,

James