disabling halt polling broken? (was Re: [PATCH 00/14] KVM: Halt-polling fixes, cleanups and a new stat)

[Sean Christopherson]

On Mon, Sep 27, 2021, Paolo Bonzini wrote:
> On Mon, Sep 27, 2021 at 5:17 PM Christian Borntraeger
> <borntraeger at de.ibm.com> wrote:
> > > So I think there are two possibilities that makes sense:
> > >
> > > * track what is using KVM_CAP_HALT_POLL, and make writes to halt_poll_ns follow that
> >
> > what about using halt_poll_ns for those VMs that did not uses KVM_CAP_HALT_POLL and the private number for those that did.
> 
> Yes, that's what I meant.  David pointed out that doesn't allow you to
> disable halt polling altogether, but for that you can always ask each
> VM's userspace one by one, or just not use KVM_CAP_HALT_POLL. (Also, I
> don't know about Google's usecase, but mine was actually more about
> using KVM_CAP_HALT_POLL to *disable* halt polling on some VMs!).

I kinda like the idea if special-casing halt_poll_ns=0, e.g. for testing or
in-the-field mitigation if halt-polling is broken.  It'd be trivial to support, e.g.

@@ -3304,19 +3304,23 @@ void kvm_vcpu_halt(struct kvm_vcpu *vcpu)
                update_halt_poll_stats(vcpu, start, poll_end, !waited);

        if (halt_poll_allowed) {
+               max_halt_poll_ns = vcpu->kvm->max_halt_poll_ns;
+               if (!max_halt_poll_ns || !halt_poll_ns)  <------ squish the max if halt_poll_ns==0
+                       max_halt_poll_ns = halt_poll_ns;
+
                if (!vcpu_valid_wakeup(vcpu)) {
                        shrink_halt_poll_ns(vcpu);
-               } else if (vcpu->kvm->max_halt_poll_ns) {
+               } else if (max_halt_poll_ns) {
                        if (halt_ns <= vcpu->halt_poll_ns)
                                ;
                        /* we had a long block, shrink polling */
                        else if (vcpu->halt_poll_ns &&
-                                halt_ns > vcpu->kvm->max_halt_poll_ns)
+                                halt_ns > max_halt_poll_ns)
                                shrink_halt_poll_ns(vcpu);
                        /* we had a short halt and our poll time is too small */
-                       else if (vcpu->halt_poll_ns < vcpu->kvm->max_halt_poll_ns &&
-                                halt_ns < vcpu->kvm->max_halt_poll_ns)
-                               grow_halt_poll_ns(vcpu);
+                       else if (vcpu->halt_poll_ns < max_halt_poll_ns &&
+                                halt_ns < max_halt_poll_ns)
+                               grow_halt_poll_ns(vcpu, max_halt_poll_ns);
                } else {
                        vcpu->halt_poll_ns = 0;
                }