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. 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.
