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

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Date:	Tue, 26 May 2009 13:00:26 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
cc:	Gregory Haskins <ghaskins@...ell.com>, kvm@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	avi@...hat.com, mtosatti@...hat.com
Subject: Re: [KVM PATCH v10] kvm: add support for irqfd

On Tue, 26 May 2009, Michael S. Tsirkin wrote:

> 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

I'm sorry, but it's not very pretty. Please find another way around.



> 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);



- Davide


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