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

[Andy Lutomirski]

On Thu, Apr 12, 2018 at 12:43 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Apr 12, 2018 at 12:27 PM, Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> 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.
>
> 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.
>
> I think that would make Andy happier too.
>

It only makes me happier if the userspace code involved is actually
going to work when single-stepped, which might actually be the case
(fingers crossed).  That being said, I'm not really convinced that
cpu_opv() makes much difference here, since I'm not entirely convinced
that user code will actually use it or that user code will actually be
that well tested.  C'est la vie.