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SubjectRe: disabling halt polling broken? (was Re: [PATCH 00/14] KVM: Halt-polling fixes, cleanups and a new stat)
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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@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;
> }
> _______________________________________________
> kvmarm mailing list
> kvmarm@lists.cs.columbia.edu
> https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
> .
Thanks,
Yanan

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