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 3:43 PM, Linus Torvalds torvalds@linux-foundation.org wrote:

> On Thu, Apr 12, 2018 at 12:27 PM, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>> The cpu_opv system call executes a vector of operations on behalf of
>> user-space on a specific CPU with preemption disabled. It is inspired
>> by readv() and writev() system calls which take a "struct iovec"
>> array as argument.
> 
> Do we really want the page pinning?
> 
> This whole cpu_opv thing is the most questionable part of the series,
> and the page pinning is the most questionable part of cpu_opv for me.

What are your concerns about page pinning ?

Do you have an alternative approach in mind ?

> 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.

The main problem with the incremental approach is that it won't deal
with remote CPU data accesses, and won't deal with cpu hotplug in
non-racy ways. For *some* of the use-cases, the other issues solved by
cpu_opv can be worked-around in user-space, at the cost of making
the userspace code a mess, and in many cases slower than if we can rely
on cpu_opv for the fallback.

All the rseq test-cases depend on cpu_opv as they stand now. Without
cpu_opv to handle the corner-cases, things become much more messy on the
user-space side.

Thanks,

Mathieu

> 
> I think that would make Andy happier too.
> 
>              Linus

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