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

[Gregory Haskins]

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?
>   

Yep.

> And this is on each interrupt. Seems like a pity.
>   

I agree.  Moving towards a way to be able to inject without deferring to
a workqueue will be a good thing.  Note, however, that addressing it at
the eventfd/wqh-lock layer is only part of the picture since ideally we
can inject (i.e. eventfd_signal()) from any atomic context (e.g.
hard-irq), not just the artificial one created by the wqh based
implementation.  I think Marcelo's irq_lock split-up code is taking us
in that direction by (eventually) allowing the kvm_set_irq() path to be
atomic-context friendly.

> 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);
>
>   

If we think we still need to address it at the eventfd layer (which I am
not 100% convinced we do), I think we should probably generalize it a
little more and make it so it doesn't completely re-route the
notification (there may be other end-points interrested in the event, I
suppose).

I am thinking something along the lines that the internal eventfd uses
an srcu_notifier, and we register a default notifier which points to a
wqh path very much like what we have today.  Then something like kvm
could register an additional srcu_notifier which should allow it to be
invoked lockless (*).  This would theoretically allow the eventfd to
remain free to support an arbitrary number of end-points which support
both locked and lockless operation.

-Greg

(*) disclaimer: I've never looked at the srcu_notifier implementation,
so perhaps this is not what they really offer.  I base this only on
basic RCU understanding.

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