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    SubjectRe: [RFC PATCH for 4.18 12/23] cpu_opv: Provide cpu_opv system call (v7)
    ----- On Apr 12, 2018, at 3:43 PM, Linus Torvalds torvalds@linux-foundation.org wrote:

    > On Thu, Apr 12, 2018 at 12:27 PM, Mathieu Desnoyers
    > <mathieu.desnoyers@efficios.com> wrote:
    >> The cpu_opv system call executes a vector of operations on behalf of
    >> user-space on a specific CPU with preemption disabled. It is inspired
    >> by readv() and writev() system calls which take a "struct iovec"
    >> array as argument.
    >
    > Do we really want the page pinning?
    >
    > This whole cpu_opv thing is the most questionable part of the series,
    > and the page pinning is the most questionable part of cpu_opv for me.

    What are your concerns about page pinning ?

    Do you have an alternative approach in mind ?

    > Can we plan on merging just the plain rseq parts *without* this all
    > first, and then see the cpu_opv thing as a "maybe future expansion"
    > part.

    The main problem with the incremental approach is that it won't deal
    with remote CPU data accesses, and won't deal with cpu hotplug in
    non-racy ways. For *some* of the use-cases, the other issues solved by
    cpu_opv can be worked-around in user-space, at the cost of making
    the userspace code a mess, and in many cases slower than if we can rely
    on cpu_opv for the fallback.

    All the rseq test-cases depend on cpu_opv as they stand now. Without
    cpu_opv to handle the corner-cases, things become much more messy on the
    user-space side.

    Thanks,

    Mathieu

    >
    > I think that would make Andy happier too.
    >
    > Linus

    --
    Mathieu Desnoyers
    EfficiOS Inc.
    http://www.efficios.com

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