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

[Claudio Imbrenda]

On Wed, 20 Oct 2021 08:03:40 +0200
Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote:

> 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.

my only concern was if things would break in case we kick the vCPU
again after clearing the bit; it seems nothing breaks, so I'm ok with it

> >   
> >>   	return kvm_s390_vcpu_has_irq(vcpu, 0);
> >>   }
> >>     
> >