Re: [PATCH] KVM: Unregister IRQ ACK notifier with in-kernel irqchip

[Sheng Yang <sheng@linux.intel.com>]

On Wednesday 08 October 2008 17:20:20 Amit Shah wrote:
> * On Wednesday 08 Oct 2008 14:34:05 Sheng Yang wrote:
> > On Wednesday 08 October 2008 16:54:18 Sheng Yang wrote:
> > > On Wednesday 08 October 2008 15:08:52 Amit Shah wrote:
> > > > * On Wednesday 08 Oct 2008 12:09:20 Sheng Yang wrote:
> > > > > Signed-off-by: Sheng Yang <sheng@linux.intel.com>
> > > > > ---
> > > > >  arch/x86/kvm/x86.c |    4 +++-
> > > > >  1 files changed, 3 insertions(+), 1 deletions(-)
> > > > >
> > > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > > > index 675fcc1..c5763d7 100644
> > > > > --- a/arch/x86/kvm/x86.c
> > > > > +++ b/arch/x86/kvm/x86.c
> > > > > @@ -176,7 +176,9 @@ static void kvm_free_assigned_device(struct kvm
> > > > > *kvm, if (irqchip_in_kernel(kvm) && assigned_dev->irq_requested)
> > > > >  		free_irq(assigned_dev->host_irq, (void *)assigned_dev);
> > > > >
> > > > > -	kvm_unregister_irq_ack_notifier(kvm,
> > > > > &assigned_dev->ack_notifier); +	if (irqchip_in_kernel(kvm))
> > > > > +		kvm_unregister_irq_ack_notifier(kvm,
> > > > > +				&assigned_dev->ack_notifier);
> > > >
> > > > The unregister API should perform the check whether the said notifier
> > > > exists so this shouldn't be necessary.
> > >
> > > Yeah, that's more reasonable. But now I just see,
> > > kvm_register_irq_ack_notifier() go with irqchip_in_kernel() and
> > > unregister didn't. :)
>
> Yes, because if we don't use the irqchip in the kernel, we don't need it at
> all.
>
> However, there's no need to special-case the unregister path as you note
> below.
>
> > Um... After consider a little more, I think keep it unwrapped by
> > irqchip_in_kernel() may be a little more reasonable. The reason is we
> > just register kvm_register_irq_ack_notifier() when we have in kernel
> > irqchip. If we don't have in kernel irqchip, we shouldn't call them and
> > try to perform this action. Call the function without in-kernel irqchip
> > is wrong, ensure the exist of in-kernel irqchip is the caller's
> > responsibility. What we can do is add a BUG_ON() or ASSERT() to the
> > function, rather let the function tell if itself need in-kernel irqchip.
>
> We ensure our notifier is registered only in the in-kernel irqchip case.
> Calling the unregister function in all the cases, though, should be fine I
> think. BUG_ON or ASSERT are conditions used to trap hardware bugs or
> unexpected paths in software. This doesn't qualify for that.

Yeah, so register should got some ASSERT. 
>
> To make the code symmetrical, it might make sense to have the unregister
> path in the kernel-irqchip case, but I really don't think it's necessary
> and doesn't hurt readability as well. Also, the lesser the conditionals we
> have the better.

OK. Seems we need a NULL judgement of unregister to keep it safe.

--
regards
Yang, Sheng
>
> Amit