Re: KVM: x86: add module parameter to disable periodic kvmclock sync

["Radim Krčmář" <rkrcmar@redhat.com>]

2014-11-13 12:32+0100, Andrew Jones:
> On Thu, Nov 13, 2014 at 11:44:02AM +0100, Andrew Jones wrote:
> > On Thu, Nov 13, 2014 at 09:40:41AM +0100, Andrew Jones wrote:
> > > On Thu, Nov 13, 2014 at 12:44:39AM -0200, Marcelo Tosatti wrote:
> > > > The periodic kvmclock sync can be an undesired source of latencies.
[...]
> > > > +static bool kvmclock_periodic_sync = 1;
> > > 
> > > Using 'true' would look nicer.
> > 
> > Ahh, disregard this comment. 1 matches what the user would input.

The output is always Y|N and the user can input that as well.
(bool = {true, false}, so I'd prefer 'true'.)

> @@ -1717,7 +1717,8 @@ static void kvmclock_sync_fn(struct work_struct *work)
>  					   kvmclock_sync_work);
>  	struct kvm *kvm = container_of(ka, struct kvm, arch);
>  
> -	schedule_delayed_work(&kvm->arch.kvmclock_update_work, 0);
> +	if (kvmclock_periodic_sync)
> +		schedule_delayed_work(&kvm->arch.kvmclock_update_work, 0);
>  	schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
>  					KVMCLOCK_SYNC_PERIOD);

Yes, or add callbacks to sysfs writes that would schedule/cancel this
work.  (But having a for_every_vm loop is quite ugly.)

I'd be happy with a 'const kvmclock_periodic_sync'.
(Having useless timers is weird if we care about latencies.)