Messages in this thread | | | Date | Fri, 18 Nov 2016 09:49:34 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH v12 6/7] x86/arch_prctl: Add ARCH_[GET|SET]_CPUID |
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On Fri, 18 Nov 2016, Ingo Molnar wrote: > * Kyle Huey <me@kylehuey.com> 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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