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    SubjectRe: [PATCH v9 29/51] mm/mprotect, powerpc/mm/pkeys, x86/mm/pkeys: Add sysfs interface
    On Tue, Dec 19, 2017 at 09:50:24PM +1100, Michael Ellerman wrote:
    > Dave Hansen <dave.hansen@intel.com> writes:
    >
    > > On 11/06/2017 12:57 AM, Ram Pai wrote:
    > >> Expose useful information for programs using memory protection keys.
    > >> Provide implementation for powerpc and x86.
    > >>
    > >> On a powerpc system with pkeys support, here is what is shown:
    > >>
    > >> $ head /sys/kernel/mm/protection_keys/*
    > >> ==> /sys/kernel/mm/protection_keys/disable_access_supported <==
    > >> true
    > >
    > > This is cute, but I don't think it should be part of the ABI. Put it in
    > > debugfs if you want it for cute tests. The stuff that this tells you
    > > can and should come from pkey_alloc() for the ABI.
    >
    > Yeah I agree this is not sysfs material.
    >
    > In particular the total/usable numbers are completely useless vs other
    > threads allocating pkeys out from under you.

    The usable number is the minimum number of keys available for use by the
    application, not the number of keys **currently** available. Its a
    static number.

    I am dropping this patch. We can revisit this when a clear request for
    such a feature emerges.

    >
    > >
    > >> Any application wanting to use protection keys needs to be able to
    > >> function without them. They might be unavailable because the
    > >> hardware that the application runs on does not support them, the
    > >> kernel code does not contain support, the kernel support has been
    > >> disabled, or because the keys have all been allocated, perhaps by a
    > >> library the application is using. It is recommended that
    > >> applications wanting to use protection keys should simply call
    > >> pkey_alloc(2) and test whether the call succeeds, instead of
    > >> attempting to detect support for the feature in any other way.
    > >
    > > Do you really not have standard way on ppc to say whether hardware
    > > features are supported by the kernel? For instance, how do you know if
    > > a given set of registers are known to and are being context-switched by
    > > the kernel?
    >
    > Yes we do, we emit feature bits in the AT_HWCAP entry of the aux vector,
    > same as some other architectures.

    Ah. I was not aware of this.
    Thanks,
    RP

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