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Date:   Thu, 23 Nov 2017 23:01:11 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <andi@...stfloor.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        linux-kernel <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>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Chris Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH for 4.15 v12 00/22] Restartable sequences and CPU op
 vector

----- On Nov 23, 2017, at 5:51 PM, Thomas Gleixner tglx@...utronix.de wrote:

> On Thu, 23 Nov 2017, Mathieu Desnoyers wrote:
>> ----- On Nov 22, 2017, at 2:37 PM, Will Deacon will.deacon@....com wrote:
>> > On Wed, Nov 22, 2017 at 08:32:19PM +0100, Peter Zijlstra wrote:
>> >>
>> >> So what exactly is the problem of leaving out the whole cpu_opv thing
>> >> for now? Pure rseq is usable -- albeit a bit cumbersome without
>> >> additional debugger support.
>> > 
>> > Drive-by "ack" to that. I'd really like a working rseq implementation in
>> > mainline, but I don't much care for another interpreter.
>> 
>> Considering the arm 64 use-case of reading PMU counters from user-space
>> using rseq to prevent migration, I understand that you're lucky enough to
>> already have a system call at your disposal that can perform the slow-path
>> in case of single-stepping.
>> 
>> So yes, your particular case is already covered, but unfortunately that's
>> not the same situation for other use-cases that have been expressed.
> 
> If we have users of rseq which can do without the other muck, then what's
> the reason not to support it?
> 
> The sysops thing can be sorted out on top and the use cases which need both
> will have to test for both syscalls being available anyway.

I'm currently making sure CONFIG_RSEQ selects both CONFIG_CPU_OPV and
CONFIG_MEMBARRIER, so the user-space fast-paths don't end up with
various ways of doing the fallback/single-stepping/memory barrier handling
depending on whether the kernel support each of those individually.
So first of all, it reduces complexity from a user-space perspective.

Moreover, with a single already needed cpu_id vs cpu_id_start field comparison
in the rseq fast-path, user-space knows that it can rely on having rseq,
cpu_opv, and membarrier. Without this guarantee, user-space would have to
detect individually whether each of those system calls is available, and
test flags on the fast-path, for additional overhead.

Those are my main concerns about pushing an incomplete solution at this
stage.

Thanks,

Mathieu

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

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