On Mon, Sep 27, 2021 at 8:17 AM Christian Borntraeger <borntraeger@xxxxxxxxxx> 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@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 (;;)". 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 > >
