Re: [PATCH 3/3] KVM: s390: clear kicked_mask if not idle after set

[Christian Borntraeger]

Am 20.10.21 um 09:52 schrieb Halil Pasic:
> On Tue, 19 Oct 2021 23:35:25 +0200
> Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote:
>
> > > @@ -426,6 +426,7 @@ static void __unset_cpu_idle(struct kvm_vcpu *vcpu)
> > >    {
> > >    	kvm_s390_clear_cpuflags(vcpu, CPUSTAT_WAIT);
> > >    	clear_bit(vcpu->vcpu_idx, vcpu->kvm->arch.idle_mask);
> > > +	clear_bit(vcpu->vcpu_idx, vcpu->kvm->arch.gisa_int.kicked_mask);
>
> BTW, do you know are bit-ops garanteed to be serialized as seen by
> another cpu even when acting on a different byte? I mean
> could the kick_single_vcpu() set the clear of the kicked_mask bit but
> not see the clear of the idle mask?

clear_bit explicitely says.
 * This is a relaxed atomic operation (no implied memory barriers).

so if we really need the ordering, then we need to add a barrier.

> If that is not true we may need some barriers, or possibly merging the
> two bitmasks like idle bit, kick bit alterating to ensure there
> absolutely ain't no race.