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

[Christian Borntraeger]

Am 27.09.21 um 17:03 schrieb Paolo Bonzini:
> On 27/09/21 16:59, Sean Christopherson wrote:
>>> commit acd05785e48c01edb2c4f4d014d28478b5f19fb5
>>> Author:     David Matlack<dmatlack at google.com>
>>> AuthorDate: Fri Apr 17 15:14:46 2020 -0700
>>> Commit:     Paolo Bonzini<pbonzini at redhat.com>
>>> CommitDate: Fri Apr 24 12:53:17 2020 -0400
>>>
>>>      kvm: add capability for halt polling
>>>
>>> broke the possibility for an admin to disable halt polling for already running KVM guests.
>>> In past times doing
>>> echo 0 > /sys/module/kvm/parameters/halt_poll_ns
>>>
>>> stopped polling system wide.
>>> Now all KVM guests will use the halt_poll_ns value that was active during
>>> startup - even those that do not use KVM_CAP_HALT_POLL.
>>>
>>> I guess this was not intended?
> 
> No, but...
> 
>> I would go so far as to say that halt_poll_ns should be a hard limit on
>> the capability
> 
> ... this would not be a good idea I think.  Anything that wants to do a lot of polling can just do "for (;;)".
> 
> 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.
> 
> * just make halt_poll_ns read-only.
> 
> Paolo
>