Re: [PATCH 4/6] KVM: Add kvm_get_irq_routing_entry() func

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

On Wednesday 17 November 2010 22:01:41 Avi Kivity wrote:
> On 11/15/2010 11:15 AM, Sheng Yang wrote:
> > We need to query the entry later.
> > 
> > +int kvm_get_irq_routing_entry(struct kvm *kvm, int gsi,
> > +		struct kvm_kernel_irq_routing_entry *entry)
> > +{
> > +	int count = 0;
> > +	struct kvm_kernel_irq_routing_entry *ei = NULL;
> > +	struct kvm_irq_routing_table *irq_rt;
> > +	struct hlist_node *n;
> > +
> > +	rcu_read_lock();
> > +	irq_rt = rcu_dereference(kvm->irq_routing);
> > +	if (gsi<  irq_rt->nr_rt_entries)
> > +		hlist_for_each_entry(ei, n,&irq_rt->map[gsi], link)
> > +			count++;
> > +	if (count == 1)
> > +		*entry = *ei;
> > +	rcu_read_unlock();
> > +
> > +	return (count != 1);
> > +}
> > +
> 
> Not good form to rely on ei being valid after the loop.
> 
> I guess this is only useful for msi?  Need to document it.

May can be used for others later, it's somehow generic. Where should I document 
it?
> 
> *entry may be stale after rcu_read_unlock().  Is this a problem?

I suppose not. All MSI-X MMIO accessing would be executed without delay, so no re-
order issue would happen. If the guest is reading and writing the field at the same 
time(from two cpus), it should got some kinds of sync method for itself - or it 
may not care what's the reading result(like the one after msix_mask_irq()). 

--
regards
Yang, Sheng