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

[David Matlack]

On Mon, Sep 27, 2021 at 8:17 AM Christian Borntraeger
<borntraeger at de.ibm.com> wrote:
>
>
>
> 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 (;;)".

I agree. It would also be a maintenance burden and subtle "gotcha" to
have to increase halt_poll_ns anytime one wants to increase
KVM_CAP_HALT_POLL.

> >
> > 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.

None of these options would cover Christian's original use-case
though. (Write to module to disable halt-polling system-wide.)

What about adding a writable "enable_halt_polling" module parameter
that affects all VMs? Once that is in place we could also consider
getting rid of halt_poll_ns entirely.

> >
> > * just make halt_poll_ns read-only.
> >
> > Paolo
> >