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Date: Mon, 16 Apr 2018 12:28:14 -0400 (EDT) From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> To: Andi Kleen <andi@...stfloor.org> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.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>, Thomas Gleixner <tglx@...utronix.de>, 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>, 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 Apr 12, 2018, at 4:23 PM, Andi Kleen andi@...stfloor.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
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