linux-kernel - Re: [KVM PATCH 1/2] KVM: Directly inject interrupts via irqfd

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Date:	Wed, 21 Oct 2009 17:36:40 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Gregory Haskins <gregory.haskins@...il.com>
Cc:	Gregory Haskins <ghaskins@...ell.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	alacrityvm-devel@...ts.sourceforge.net
Subject: Re: [KVM PATCH 1/2] KVM: Directly inject interrupts via irqfd

On Wed, Oct 21, 2009 at 11:34:51AM -0400, Gregory Haskins wrote:
> Gleb Natapov wrote:
> > On Wed, Oct 21, 2009 at 10:34:53AM -0400, Gregory Haskins wrote:
> >> IRQFD currently uses a deferred workqueue item to execute the injection
> >> operation.  It was originally designed this way because kvm_set_irq()
> >> required the caller to hold the irq_lock mutex, and the eventfd callback
> >> is invoked from within a non-preemptible critical section.
> >>
> >> With the advent of lockless injection support in kvm_set_irq, the deferment
> >> mechanism is no longer technically needed. Since context switching to the
> >> workqueue is a source of interrupt latency, lets switch to a direct
> >> method.
> >>
> > kvm_set_irq is fully lockless only in MSI case. IOAPIC/PIC has mutexes.
> 
> Right, but irqfd by design only works with MSI (or MSI like edge
> triggers) anyway.  Legacy-type injections follow a different path.
> 
Ah, If this the case and it will stay that way then the change looks OK
to me.

> In any case, I didn't change the locking (you did ;).  You recently
> patched the irqfd code to remove the irq_lock, but we still had the
> deferment mechanism in place to avoid the mutex_lock from within the
> POLLIN callback.  Since the mutex_lock is now no longer acquired in this
> path, the deferment technique is not needed either.  Its only adding
> overhead for no purpose.  So I am simply cleaning that up to improve
> interrupt performance.
> 
> HTH,
> -Greg
> 
> 



--
			Gleb.
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