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Date:   Wed, 28 Mar 2018 14:18:11 -0700 (PDT)
From:   David Rientjes <rientjes@google.com>
To:     Laurent Dufour <ldufour@linux.vnet.ibm.com>
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Subject: Re: [PATCH v9 01/24] mm: Introduce CONFIG_SPECULATIVE_PAGE_FAULT
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On Wed, 28 Mar 2018, Laurent Dufour wrote:

> > Putting this in mm/Kconfig is definitely the right way to go about it 
> > instead of any generic option in arch/*.
> > 
> > My question, though, was making this configurable by the user:
> > 
> > config SPECULATIVE_PAGE_FAULT
> > 	bool "Speculative page faults"
> > 	depends on X86_64 || PPC
> > 	default y
> > 	help
> > 	  ..
> > 
> > It's a question about whether we want this always enabled on x86_64 and 
> > power or whether the user should be able to disable it (right now they 
> > can't).  With a large feature like this, you may want to offer something 
> > simple (disable CONFIG_SPECULATIVE_PAGE_FAULT) if someone runs into 
> > regressions.
> 
> I agree, but I think it would be important to get the per architecture
> enablement to avoid complex check here. For instance in the case of powerPC
> this is only supported for PPC_BOOK3S_64.
> 
> To avoid exposing such per architecture define here, what do you think about
> having supporting architectures setting ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT
> and the SPECULATIVE_PAGE_FAULT depends on this, like this:
> 
> In mm/Kconfig:
> config SPECULATIVE_PAGE_FAULT
>  	bool "Speculative page faults"
>  	depends on ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT && SMP
>  	default y
>  	help
> 		...
> 
> In arch/powerpc/Kconfig:
> config PPC
> 	...
> 	select ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT	if PPC_BOOK3S_64
> 
> In arch/x86/Kconfig:
> config X86_64
> 	...
> 	select ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT
> 
> 

Looks good to me!  It feels like this will add more assurance that if 
things regress for certain workloads that it can be disabled.  I don't 
feel strongly about the default value, I'm ok with it being enabled by 
default.