linux-kernel - Re: [PATCH RFC] kvm: deliver msix interrupts from irq handler

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Date:	Thu, 19 Jan 2012 17:57:48 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Alex Williamson <alex.williamson@...hat.com>,
	jan.kiszka@...mens.com, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] kvm: deliver msix interrupts from irq handler

On Thu, Jan 19, 2012 at 05:02:17PM +0200, Gleb Natapov wrote:
> 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.

No, the common code is the surrounding foreach loop,
the internal if branch is different.

> > - 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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