On 28/09/19 19:23, Andrea Arcangeli wrote: > Reducing this list to only EXIT_REASON_MSR_WRITE, > EXIT_REASON_PREEMPTION_TIMER, EXIT_REASON_EPT_MISCONFIG, > EXIT_REASON_IO_INSTRUCTION increases the computation time of the > hrtimer guest testcase on Haswell i5-4670T CPU @ 2.30GHz by 7% with > the default spectre v2 mitigation enabled in the host and guest. On > skylake as opposed there's no measurable difference with the short > list. To put things in prospective on Haswell the same hrtimer > workload (note: it never calls cpuid and it never attempts to trigger > more vmexit on purpose) in guest takes 16.3% longer to compute on > upstream KVM running in the host than with the KVM mono v1 patchset > applied to the host kernel, while on skylake the same takes only 5.4% > more time (both with the default mitigations enabled in guest and > host). > > It's also unclear why EXIT_REASON_IO_INSTRUCTION should be included. If you're including EXIT_REASON_EPT_MISCONFIG (MMIO access) then you should include EXIT_REASON_IO_INSTRUCTION too. Depending on the devices that are in the guest, the doorbell register might be MMIO or PIO. > + if (exit_reason == EXIT_REASON_MSR_WRITE) > + return kvm_emulate_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 kvm_emulate_halt(vcpu); > + else if (exit_reason == EXIT_REASON_PAUSE_INSTRUCTION) > + return handle_pause(vcpu); > + else if (exit_reason == EXIT_REASON_MSR_READ) > + return kvm_emulate_rdmsr(vcpu); > + else if (exit_reason == EXIT_REASON_CPUID) > + return kvm_emulate_cpuid(vcpu); > + else if (exit_reason == EXIT_REASON_EPT_MISCONFIG) > + return handle_ept_misconfig(vcpu); So, the difference between my suggested list (which I admit is just based on conjecture, not benchmarking) is that you add EXIT_REASON_PAUSE_INSTRUCTION, EXIT_REASON_PENDING_INTERRUPT, EXIT_REASON_EXTERNAL_INTERRUPT, EXIT_REASON_HLT, EXIT_REASON_MSR_READ, EXIT_REASON_CPUID. Which of these make a difference for the hrtimer testcase? It's of course totally fine to use benchmarks to prove that my intuition was bad---but you must also use them to show why your intuition is right. :) Paolo
