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SubjectRe: [PATCH v9 01/24] mm: Introduce CONFIG_SPECULATIVE_PAGE_FAULT
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On 28/03/2018 12:16, David Rientjes wrote:
> On Wed, 28 Mar 2018, Laurent Dufour wrote:
>
>>>> This configuration variable will be used to build the code needed to
>>>> handle speculative page fault.
>>>>
>>>> By default it is turned off, and activated depending on architecture
>>>> support.
>>>>
>>>> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
>>>> Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
>>>> ---
>>>> mm/Kconfig | 3 +++
>>>> 1 file changed, 3 insertions(+)
>>>>
>>>> diff --git a/mm/Kconfig b/mm/Kconfig
>>>> index abefa573bcd8..07c566c88faf 100644
>>>> --- a/mm/Kconfig
>>>> +++ b/mm/Kconfig
>>>> @@ -759,3 +759,6 @@ config GUP_BENCHMARK
>>>> performance of get_user_pages_fast().
>>>>
>>>> See tools/testing/selftests/vm/gup_benchmark.c
>>>> +
>>>> +config SPECULATIVE_PAGE_FAULT
>>>> + bool
>>>
>>> Should this be configurable even if the arch supports it?
>>
>> Actually, this is not configurable unless by manually editing the .config file.
>>
>> I made it this way on the Thomas's request :
>> https://lkml.org/lkml/2018/1/15/969
>>
>> That sounds to be the smarter way to achieve that, isn't it ?
>>
>
> 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

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