Re: [PATCH 1/3] KVM: s390: clear kicked_mask before sleeping again

[Christian Borntraeger]

Am 20.10.21 um 07:35 schrieb Claudio Imbrenda:
> On Tue, 19 Oct 2021 19:53:59 +0200
> Halil Pasic <pasic@xxxxxxxxxxxxx> wrote:
>
> > The idea behind kicked mask is that we should not re-kick a vcpu that
> > is already in the "kick" process, i.e. that was kicked and is
> > is about to be dispatched if certain conditions are met.
> >
> > The problem with the current implementation is, that it assumes the
> > kicked vcpu is going to enter SIE shortly. But under certain
> > circumstances, the vcpu we just kicked will be deemed non-runnable and
> > will remain in wait state. This can happen, if the interrupt(s) this
> > vcpu got kicked to deal with got already cleared (because the interrupts
> > got delivered to another vcpu). In this case kvm_arch_vcpu_runnable()
> > would return false, and the vcpu would remain in kvm_vcpu_block(),
> > but this time with its kicked_mask bit set. So next time around we
> > wouldn't kick the vcpu form __airqs_kick_single_vcpu(), but would assume
> > that we just kicked it.
> >
> > Let us make sure the kicked_mask is cleared before we give up on
> > re-dispatching the vcpu.
> >
> > Signed-off-by: Halil Pasic <pasic@xxxxxxxxxxxxx>
> > Reported-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
> > Fixes: 9f30f6216378 ("KVM: s390: add gib_alert_irq_handler()")
> > ---
> >   arch/s390/kvm/kvm-s390.c | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> > index 6a6dd5e1daf6..1c97493d21e1 100644
> > --- a/arch/s390/kvm/kvm-s390.c
> > +++ b/arch/s390/kvm/kvm-s390.c
> > @@ -3363,6 +3363,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> >   
> >   int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
> >   {
> > +	clear_bit(vcpu->vcpu_idx, vcpu->kvm->arch.gisa_int.kicked_mask);
>
> so, you unconditionally clear the flag, before knowing if the vCPU is
> runnable?
>
> from your description I would have expected to only clear the bit if
> the vCPU is not runnable.
>
> would things break if we were to try to kick the vCPU again after
> clearing the bit, but before dispatching it?

The whole logic is just an optimization to avoid unnecessary wakeups.
When the bit is set a wakup might be omitted.
I prefer to do an unneeded wakeup over not doing a wakeup so I think
over-clearing is safer.
In fact, getting rid of this micro-optimization would be a valid
alternative.
> >   	return kvm_s390_vcpu_has_irq(vcpu, 0);
> >   }
> >