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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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