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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Sat, 14 Apr 2018 15:44:06 -0700
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Subject: Re: [RFC PATCH for 4.18 12/23] cpu_opv: Provide cpu_opv system call (v7)
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Paul Turner <pjt@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
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        Andi Kleen <andi@firstfloor.org>, Chris Lameter <cl@linux.com>,
        Ben Maurer <bmaurer@fb.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>
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On Thu, Apr 12, 2018 at 12:43 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.
>
> 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.