Re: [RFC PATCH for 4.18 12/23] cpu_opv: Provide cpu_opv system call (v7)

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- 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