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SubjectRe: [PATCH RFC] kvm: deliver msix interrupts from irq handler
On Thu, Jan 19, 2012 at 03:49:57PM +0200, Michael S. Tsirkin wrote:
> > > + irq_rt = rcu_dereference(kvm->irq_routing);
> > > + if (irq < irq_rt->nr_rt_entries)
> > > + hlist_for_each_entry(e, n, &irq_rt->map[irq], link) {
> > > + if (ei->type == KVM_IRQ_ROUTING_MSI)
> > > + ret = kvm_set_msi(e, kvm, irq_source_id, level,
> > > + host_irq);
> > > + else
> > > + ret = -EWOULDBLOCK;
> > > + break;
> > > + }
> > > + rcu_read_unlock_bh();
> > > + return ret;
> > > +}
> > > +
> > Share implementation with kvm_set_irq().
>
> I considered this. There are several reasons not to do it:
> - Amount of common code is very small
Why? Just pass msi_only flag to kvm_set_irq() and skip an entry if flag is
set and entry type is not msi.

> - As it's separate, it's more obvious that it can't block (kvm_set_irq can block)
> We can even tag kvm_set_irq with might_sleep.
They can still be two separate function calling common one.

> - This is way simpler and faster as we can do operations directly,
> instead of copying the irq out, and as it's datapath
> an optimization is I think justified.
I really do not think the copy of one small data structure will be
measurable. If it is (has to be proven) we can optimize that two
in the common code.

--
Gleb.


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