On Mon, Sep 23, 2019 at 03:05:14PM -0400, Andrea Arcangeli wrote: > On Mon, Sep 23, 2019 at 11:57:57AM +0200, Paolo Bonzini wrote: > > On 23/09/19 11:31, Vitaly Kuznetsov wrote: > > > +#ifdef CONFIG_RETPOLINE > > > + if (exit_reason == EXIT_REASON_MSR_WRITE) > > > + return handle_wrmsr(vcpu); > > > + else if (exit_reason == EXIT_REASON_PREEMPTION_TIMER) > > > + return handle_preemption_timer(vcpu); > > > + else if (exit_reason == EXIT_REASON_PENDING_INTERRUPT) > > > + return handle_interrupt_window(vcpu); > > > + else if (exit_reason == EXIT_REASON_EXTERNAL_INTERRUPT) > > > + return handle_external_interrupt(vcpu); > > > + else if (exit_reason == EXIT_REASON_HLT) > > > + return handle_halt(vcpu); > > > + else if (exit_reason == EXIT_REASON_PAUSE_INSTRUCTION) > > > + return handle_pause(vcpu); > > > + else if (exit_reason == EXIT_REASON_MSR_READ) > > > + return handle_rdmsr(vcpu); > > > + else if (exit_reason == EXIT_REASON_CPUID) > > > + return handle_cpuid(vcpu); > > > + else if (exit_reason == EXIT_REASON_EPT_MISCONFIG) > > > + return handle_ept_misconfig(vcpu); > > > +#endif > > > return kvm_vmx_exit_handlers[exit_reason](vcpu); > > > > Most of these, while frequent, are already part of slow paths. > > > > I would keep only EXIT_REASON_MSR_WRITE, EXIT_REASON_PREEMPTION_TIMER, > > EXIT_REASON_EPT_MISCONFIG and add EXIT_REASON_IO_INSTRUCTION. > > Intuition doesn't work great when it comes to CPU speculative > execution runtime. I can however run additional benchmarks to verify > your theory that keeping around frequent retpolines will still perform > ok. On one most recent CPU model there's no measurable difference with your list or my list with a hrtimer workload (no cpuid). It's challenging to measure any difference below 0.5%. An artificial cpuid loop takes 1.5% longer to compute if I don't add CPUID to the list, but that's just the measurement of the cost of hitting a frequent retpoline in the exit reason handling code. Thanks, Andrea
