Re: 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@xxxxxxxxxx>
> > > AuthorDate: Fri Apr 17 15:14:46 2020 -0700
> > > Commit:     Paolo Bonzini<pbonzini@xxxxxxxxxx>
> > > 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