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Date:   Mon, 16 Apr 2018 12:28:14 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Andi Kleen <andi@firstfloor.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Dave Watson <davejwatson@fb.com>,
        linux-kernel <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>,
        "H. Peter Anvin" <hpa@zytor.com>, Andrew Hunter <ahh@google.com>,
        Chris Lameter <cl@linux.com>, Ben Maurer <bmaurer@fb.com>,
        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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Subject: Re: [RFC PATCH for 4.18 12/23] cpu_opv: Provide cpu_opv system call
 (v7)
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Thread-Topic: cpu_opv: Provide cpu_opv system call (v7)
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----- On Apr 12, 2018, at 4:23 PM, Andi Kleen andi@firstfloor.org wrote:

>> 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.
> 
> That would be the right way to go. I doubt anybody really needs cpu_opv.
> We already have other code (e.g. vgettimeofday) which cannot
> be single stepped, and so far it never was a problem.

Single-stepping is only a subset of the rseq limitations addressed
by cpu_opv. Anoher major limitation is algorithms requiring data
migration between per-cpu data structures safely against CPU hotplug,
and without having to change the cpu affinity mask. This is the case
for memory allocators and userspace task schedulers which require
cpu_opv for migration between per-cpu memory pools and scheduler
runqueues.

About the vgettimeofday and general handling of vDSO by gdb, gdb's
approach only takes care of line-by-line single-stepping by hiding
Linux' vdso mapping so users cannot target source code lines within
that shared object. However, it breaks instruction-level single-stepping.
I reported this issue to you back in Nov. 2017:
https://lkml.org/lkml/2017/11/20/803

Thanks,

Mathieu

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