On Fri, 18 Nov 2016, Ingo Molnar wrote:
> * Kyle Huey <me@xxxxxxxxxxxx> 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. 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.
Thanks,
tglx
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