Re: [RFCv2 03/10] x86: Introduce userspace API to handle per-thread features

Edgecombe, Rick P
2022-05-15T18:24:13Z

On Sun, 2022-05-15 at 11:02 +0200, Thomas Gleixner wrote:
> > Otherwise, the option that used to be used here was a "status"
> > arch_prctl(), which was called separately to find out what actually
> > got
> > enabled after an "enable" call. That fit with the GET/SET semantics
> > already in place.
> >
> > I guess we could also get rid of the batch enabling idea, and just
> > have
> > one "enable" call per feature too. But then it is syscall heavy.
>
> This is not a runtime hotpath problem. Those prctls() happen once
> when
> the process starts, so having three which are designed for the
> individual purpose instead of one ill defined is definitely the
> better
> choice.
>
> Premature optimization is never a good idea. Keep it simple is the
> right
> starting point.
>
> If it really turns out to be something which matters, then you can
> provide a batch interface later on if it makes sense to do so, but
> see
> above.

Thanks, sounds good to me.

Kirill, so I guess we can just change ARCH_THREAD_FEATURE_ENABLE/
ARCH_THREAD_FEATURE_DISABLE to return EINVAL if more than one bit is
set. It returns 0 on success and whatever error code on failure.
Userspace can do whatever rollback logic it wants. What do you think?