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

[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

* just make halt_poll_ns read-only.

Paolo