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

[wangyanan (Y)]

Hi Sean, Paolo,

I recently also notice the behavior change of param halt_poll_ns.
Now it loses the ability to:
1) dynamically disable halt polling for all the running VMs
by `echo 0 > /sys`
2) dynamically adjust the halt polling interval for all the
running VMs by `echo * > /sys`

While in our cases, we usually use above two abilities, and
KVM_CAP_HALT_POLL is not used yet.

On 2021/9/28 1:33, Sean Christopherson wrote:
> 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.
Do we have any plan to repost the diff as a fix?
I would be very nice that this issue can be solved.

Besides, I think we may need some Doc for users to describe
how halt_poll_ns works with KVM_CAP_HALT_POLL, like
"Documentation/virt/guest-halt-polling.rst".
> @@ -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;
> +
Does this mean that KVM_CAP_HALT_POLL will not be able to
disable halt polling for a VM individually when halt_poll_ns !=0?
>                  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;
>                  }
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Thanks,
Yanan