On 10/11/2017 11:31, Peter Zijlstra wrote: > On Fri, Nov 10, 2017 at 11:15:06AM +0100, Paolo Bonzini wrote: >> On 10/11/2017 11:08, Peter Zijlstra wrote: >>> On Fri, Nov 10, 2017 at 01:49:47AM -0800, Wanpeng Li wrote: >>>> @@ -2887,7 +2899,7 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu) >>>> if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED)) >>>> return; >>>> >>>> - vcpu->arch.st.steal.preempted = KVM_VCPU_PREEMPTED; >>>> + vcpu->arch.st.steal.preempted |= KVM_VCPU_PREEMPTED; >>> >>> I don't understand this one... If there is concurrency its wrong, if >>> there is no concurrency it still doesn't make sense as there should not >>> be any FLUSH flags to preserve.. >> >> There is no concurrency, foreign VCPUs are not going to write to the >> location unless PREEMPTED is set. So indeed the "|=" is pointless. >> >> However, I wonder if it'd be useful for a VCPU to set the bit _on >> itself_ before going to sleep. Like >> >> set KVM_VCPU_SHOULD_FLUSH >> hlt >> /* Automagic TLB flush! */ >> >> This would not work currently, but if it *is* useful, we should make it >> work and document it as legal. Peter, do you think it would make any sense? > > Almost but not quite I think.. So there is no guarantee HLT (or even > MWAIT with a state that has CPUILDE_FLAG_TLB_FLUSHED set) will actually > do the TLB flush. Well, for virt you could always guarantee it if it's useful. But from the rest of your message it looks like it would be a separate PV feature than this one. Thanks, Paolo > And if we preempt the vCPU to run a kernel thread we will not in fact > invalidate the TLBs either. > > Also, you're confusing the SHOULD_FLUSH with the HAS_FLUSHED concept. > Because if we didn't flush and we should have we should still issue it > on VMENTER. > > So if we could somehow tell if a HLT or preemption did indeed flush the > TLBs post fact (reading back the attained C state is possible but really > rather expensive IIRC), then we could set a HAS_FLUSHED flag and avoid > issuing when SHOULD_FLUSH is also set. > >
