linux-kernel - Re: [KVM PATCH v10] kvm: add support for irqfd

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Date:	Tue, 26 May 2009 19:42:01 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com,
	davidel@...ilserver.org, mtosatti@...hat.com
Subject: Re: [KVM PATCH v10] kvm: add support for irqfd

On Wed, May 20, 2009 at 10:30:49AM -0400, Gregory Haskins wrote:
> +static int
> +irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
> +{
> +	struct _irqfd *irqfd = container_of(wait, struct _irqfd, wait);
> +
> +	/*
> +	 * The wake_up is called with interrupts disabled.  Therefore we need
> +	 * to defer the IRQ injection until later since we need to acquire the
> +	 * kvm->lock to do so.
> +	 */
> +	schedule_work(&irqfd->work);
> +
> +	return 0;
> +}

This schedule_work is there just to work around the spinlock
in eventfd_signal, which we don't really need. Isn't this right?
And this is on each interrupt. Seems like a pity.
How about a flag in eventfd that would
convert it from waking up someone to a plain function call?

Davide, could we add something like


diff --git a/fs/eventfd.c b/fs/eventfd.c
index 2a701d5..8bfa308 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -29,6 +29,7 @@ struct eventfd_ctx {
 	 */
 	__u64 count;
 	unsigned int flags;
+	int nolock;
 };
 
 /*
@@ -46,6 +47,12 @@ int eventfd_signal(struct file *file, int n)
 
 	if (n < 0)
 		return -EINVAL;
+	if (ctx->nolock) {
+               /* Whoever set nolock
+                  better set wqh.func as well. */
+		ctx->wqh.func(&ctx->wqh, 0, 0, NULL);
+		return 0;
+	}
 	spin_lock_irqsave(&ctx->wqh.lock, flags);
 	if (ULLONG_MAX - ctx->count < n)
 		n = (int) (ULLONG_MAX - ctx->count);

-- 
MST
--
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