Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 22 Nov 2016 09:26:11 -0800 | Subject | Re: [PATCH v12 6/7] x86/arch_prctl: Add ARCH_[GET|SET]_CPUID |
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On Nov 21, 2016 12:27 AM, "Ingo Molnar" <mingo@kernel.org> wrote: > > > * Thomas Gleixner <tglx@linutronix.de> wrote: > > > 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. > > 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 don't see what problem we're trying to solve here. If we end up with a mix of global (and changeable!) features and per-task features, we can just do:
wrmsrl(MSR_MISC_FEATURES_ENABLES, global_misc_features_val | next_p->thread.misc_features_val);
This is *still* way faster than rdmsr.
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