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Subject: Re: [PATCH 1/3] KVM: s390: clear kicked_mask before sleeping again
To:     Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Halil Pasic <pasic@linux.ibm.com>
Cc:     Janosch Frank <frankja@linux.ibm.com>,
        Michael Mueller <mimu@linux.ibm.com>,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        Matthew Rosato <mjrosato@linux.ibm.com>,
        David Hildenbrand <david@redhat.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Pierre Morel <pmorel@linux.ibm.com>,
        Tony Krowiak <akrowiak@linux.ibm.com>,
        Niklas Schnelle <schnelle@linux.ibm.com>, farman@linux.ibm.com,
        kvm@vger.kernel.org
References: <20211019175401.3757927-1-pasic@linux.ibm.com>
 <20211019175401.3757927-2-pasic@linux.ibm.com>
 <20211020073515.3ad4c377@p-imbrenda>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
Message-ID: <1641267f-3a23-aba1-ab50-6f7c15e44528@de.ibm.com>
Date:   Wed, 20 Oct 2021 08:03:40 +0200
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Am 20.10.21 um 07:35 schrieb Claudio Imbrenda:
> On Tue, 19 Oct 2021 19:53:59 +0200
> Halil Pasic <pasic@linux.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@linux.ibm.com>
>> Reported-by: Matthew Rosato <mjrosato@linux.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?

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);
>>   }
>>   
>