* Thomas Gleixner <[email protected]> wrote:
> On Fri, 18 Nov 2016, Ingo Molnar wrote:
> > * Kyle Huey <[email protected]> wrote:
> > > + if (test_tsk_thread_flag(prev_p, TIF_NOCPUID) ^
> > > + test_tsk_thread_flag(next_p, TIF_NOCPUID)) {
> > > + set_cpuid_faulting(test_tsk_thread_flag(next_p, TIF_NOCPUID));
> > > + }
> > > +
> >
> > Why not cache the required MSR value in the task struct instead?
> >
> > That would allow something much more obvious and much faster, like:
> >
> > if (prev_p->thread.misc_features_val !=
> > next_p->thread.misc_features_val)
> > wrmsrl(MSR_MISC_FEATURES_ENABLES,
> > next_p->thread.misc_features_val);
> >
> > (The TIF flag maintenance is still required to get into __switch_to_xtra().)
> >
> > It would also be easy to extend without extra overhead, should any other
> > feature
> > bit be added to the MSR in the future.
>
> I doubt that. There are feature enable bits coming up which are not related
> to
> tasks.
Any inefficiencies resulting from such features should IMHO be carried by those
features, not by per task features - but:
> [...] So if we have switches enabling/disabling global features, then we
> would
> be forced to chase all threads in order to update all misc_features thread
> variables. Surely not what we want to do.
What switches would those be? We generally don't twiddle global CPU features
post
bootup - we pick a model on bootup and go with that.
I'd really like to see code (prototype patches are OK - or the person doing it
can
send it to me privately as well if it's not production quality or public yet),
or
some careful description of the features involved.
Thanks,
Ingo
