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

Davide Libenzi
2009-05-26T20:06:37Z

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