linux-kernel - Re: [PATCH 1/3] KVM: s390: clear kicked

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Date:   Wed, 20 Oct 2021 07:35:15 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Michael Mueller <mimu@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>,
        Tony Krowiak <akrowiak@...ux.ibm.com>,
        Niklas Schnelle <schnelle@...ux.ibm.com>, farman@...ux.ibm.com,
        kvm@...r.kernel.org
Subject: Re: [PATCH 1/3] KVM: s390: clear kicked_mask before sleeping again

On Tue, 19 Oct 2021 19:53:59 +0200
Halil Pasic <pasic@...ux.ibm.com> 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@...ux.ibm.com>
> Reported-by: Matthew Rosato <mjrosato@...ux.ibm.com>
> 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?

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