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

[Joao Martins <joao.m.martins@oracle.com>]

On 6/6/19 7:36 PM, 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.
> 

Hence why I was suggesting a *guest* cpuidle-haltpoll module parameter to still
allow it to load or otherwise (or allow guest to pick).

> 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.
> 
It's just with the new driver we unilaterally poll on both sides, just felt I
would point it out should this raise unattended performance side effects ;)

> The warning however sounds sensible.
> 

Cool.

	Joao