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Date:   Mon, 16 Apr 2018 10:02:41 -0700
From:   Andi Kleen <andi@firstfloor.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Andi Kleen <andi@firstfloor.org>,
        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>
Subject: Re: [RFC PATCH for 4.18 12/23] cpu_opv: Provide cpu_opv system call
 (v7)
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> 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

And how many people are going to implement such a complex separate
path just for CPU hotplug? And even if they implement it how long
before it bitrots? Seems more like a checkbox item than a realistic
approach.

> for memory allocators and userspace task schedulers which require
> cpu_opv for migration between per-cpu memory pools and scheduler
> runqueues.

Not sure about that. Is that common?

> 
> 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

It was known from day 1, but afaik never a problem.

-Andi