linux-kernel - Re: [RFC PATCH for 4.18 12/23] cpu_opv: Provide cpu

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Date:   Sat, 14 Apr 2018 15:44:06 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Dave Watson <davejwatson@...com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH for 4.18 12/23] cpu_opv: Provide cpu_opv system call (v7)

On Thu, Apr 12, 2018 at 12:43 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Apr 12, 2018 at 12:27 PM, Mathieu Desnoyers
> <mathieu.desnoyers@...icios.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.
>
> 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.
>
> I think that would make Andy happier too.
>

It only makes me happier if the userspace code involved is actually
going to work when single-stepped, which might actually be the case
(fingers crossed).  That being said, I'm not really convinced that
cpu_opv() makes much difference here, since I'm not entirely convinced
that user code will actually use it or that user code will actually be
that well tested.  C'est la vie.