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

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Date:	Wed, 21 Oct 2009 17:26:21 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	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 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.

> Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
> ---
> 
>  virt/kvm/eventfd.c |   15 +++------------
>  1 files changed, 3 insertions(+), 12 deletions(-)
> 
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index 30f70fd..1a529d4 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -49,16 +49,14 @@ struct _irqfd {
>  	poll_table                pt;
>  	wait_queue_head_t        *wqh;
>  	wait_queue_t              wait;
> -	struct work_struct        inject;
>  	struct work_struct        shutdown;
>  };
>  
>  static struct workqueue_struct *irqfd_cleanup_wq;
>  
>  static void
> -irqfd_inject(struct work_struct *work)
> +irqfd_inject(struct _irqfd *irqfd)
>  {
> -	struct _irqfd *irqfd = container_of(work, struct _irqfd, inject);
>  	struct kvm *kvm = irqfd->kvm;
>  
>  	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 1);
> @@ -80,12 +78,6 @@ irqfd_shutdown(struct work_struct *work)
>  	remove_wait_queue(irqfd->wqh, &irqfd->wait);
>  
>  	/*
> -	 * We know no new events will be scheduled at this point, so block
> -	 * until all previously outstanding events have completed
> -	 */
> -	flush_work(&irqfd->inject);
> -
> -	/*
>  	 * It is now safe to release the object's resources
>  	 */
>  	eventfd_ctx_put(irqfd->eventfd);
> @@ -126,7 +118,7 @@ irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
>  
>  	if (flags & POLLIN)
>  		/* An event has been signaled, inject an interrupt */
> -		schedule_work(&irqfd->inject);
> +		irqfd_inject(irqfd);
>  
>  	if (flags & POLLHUP) {
>  		/* The eventfd is closing, detach from KVM */
> @@ -179,7 +171,6 @@ kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
>  	irqfd->kvm = kvm;
>  	irqfd->gsi = gsi;
>  	INIT_LIST_HEAD(&irqfd->list);
> -	INIT_WORK(&irqfd->inject, irqfd_inject);
>  	INIT_WORK(&irqfd->shutdown, irqfd_shutdown);
>  
>  	file = eventfd_fget(fd);
> @@ -214,7 +205,7 @@ kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
>  	 * before we registered, and trigger it as if we didn't miss it.
>  	 */
>  	if (events & POLLIN)
> -		schedule_work(&irqfd->inject);
> +		irqfd_inject(irqfd);
>  
>  	/*
>  	 * do not drop the file until the irqfd is fully initialized, otherwise
> 
> --
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--
			Gleb.
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