Re: [patch v2 3/3] cpuidle-haltpoll: disable host side polling when kvm virtualized

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Jun 06, 2019 at 02:36:32PM -0400, Andrea Arcangeli wrote:
> Hello,
> 
> On Thu, Jun 06, 2019 at 07:25:28PM +0100, Joao Martins wrote:
> > But I wonder whether we should fail to load cpuidle-haltpoll when host halt
> > polling can't be disabled[*]? That is to avoid polling in both host and guest
> > and *possibly* avoid chances for performance regressions when running on older
> > hypervisors?
> 
> I don't think it's necessary: that would force an upgrade of the host
> KVM version in order to use the guest haltpoll feature with an
> upgraded guest kernel that can use the guest haltpoll.
> 
> The guest haltpoll is self contained in the guest, so there's no
> reason to prevent that by design or to force upgrade of the KVM host
> version. It'd be more than enough to reload kvm.ko in the host with
> the host haltpoll set to zero with the module parameter already
> available, to achieve the same runtime without requiring a forced host
> upgrade.
> 
> The warning however sounds sensible.

Alright, added a warning:

void arch_haltpoll_enable(void)
{
        if (!kvm_para_has_feature(KVM_FEATURE_POLL_CONTROL)) {
                WARN_ONCE(1, "kvm: host does not support poll control, can't "
                             "disable host polling (host upgrade "
                             " recommended).\n"
                return;
        }

Thanks