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

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Date:   Wed, 20 Oct 2021 10:14:50 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Claudio Imbrenda <imbrenda@...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, Halil Pasic <pasic@...ux.ibm.com>
Subject: Re: [PATCH 1/3] KVM: s390: clear kicked_mask before sleeping again

On Wed, 20 Oct 2021 08:08:16 +0200
Claudio Imbrenda <imbrenda@...ux.ibm.com> wrote:

> > >> +	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?

Right. I talked about this with  Mimu. It would extend the section
guarded by the bit, and than may be a good thing. Maybe we should
measure that alternative as well.

> > > 
> > > 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

I'm not sure about the exact impact of over-waking.
kvm_s390_vcpu_wakeup() sets vcpu->valid_wakeup which is I believe used
for some halt poll heuristics. We unset that in
kvm_arch_vcpu_block_finish(). If we cleared only conditionally the
protection would extend for that as well. Which would be a good thing.
The statistics stuff in kvm_vcpu_wake_up() does account for already
running, so I see no correctness issues there.

Regards,
Halil