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

[Andi Kleen]

> 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