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

[Avi Kivity <avi@redhat.com>]

On 11/18/2010 04:22 AM, Sheng Yang wrote:
> 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?

Non-msi interrupts (wires) can be wired to more than one interrupt line 
(and often are - pic/ioapic).

You can document it by adding _msi to the name.

> >
> >  *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()).

I guess so.  Michael/Alex?

-- 
error compiling committee.c: too many arguments to function