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

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Date:	Wed, 21 Oct 2009 10:34:53 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	kvm@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org,
	alacrityvm-devel@...ts.sourceforge.net
Subject: [KVM PATCH 1/2] KVM: Directly inject interrupts via irqfd

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.

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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