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

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Date:	Wed, 21 Oct 2009 11:34:51 -0400
From:	Gregory Haskins <gregory.haskins@...il.com>
To:	Gleb Natapov <gleb@...hat.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

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.

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

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