linux-kernel - Re: [PATCH v2 4/6] kvm: Extend irqfd to support level interrupts

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Date:	Thu, 28 Jun 2012 11:48:40 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Alex Williamson <alex.williamson@...hat.com>, avi@...hat.com,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	jan.kiszka@...mens.com
Subject: Re: [PATCH v2 4/6] kvm: Extend irqfd to support level interrupts

On Thu, Jun 28, 2012 at 11:46:11AM +0300, Gleb Natapov wrote:
> On Thu, Jun 28, 2012 at 11:41:05AM +0300, Michael S. Tsirkin wrote:
> > On Thu, Jun 28, 2012 at 11:35:41AM +0300, Gleb Natapov wrote:
> > > On Thu, Jun 28, 2012 at 11:34:35AM +0300, Michael S. Tsirkin wrote:
> > > > On Thu, Jun 28, 2012 at 09:34:31AM +0300, Gleb Natapov wrote:
> > > > > On Thu, Jun 28, 2012 at 01:31:29AM +0300, Michael S. Tsirkin wrote:
> > > > > > On Wed, Jun 27, 2012 at 04:04:18PM -0600, Alex Williamson wrote:
> > > > > > > On Wed, 2012-06-27 at 18:26 +0300, Michael S. Tsirkin wrote:
> > > > > > > > On Tue, Jun 26, 2012 at 11:09:46PM -0600, Alex Williamson wrote:
> > > > > > > > > @@ -71,6 +130,14 @@ irqfd_inject(struct work_struct *work)
> > > > > > > > >  	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 0);
> > > > > > > > >  }
> > > > > > > > >  
> > > > > > > > > +static void
> > > > > > > > > +irqfd_inject_level(struct work_struct *work)
> > > > > > > > > +{
> > > > > > > > > +	struct _irqfd *irqfd = container_of(work, struct _irqfd, inject);
> > > > > > > > > +
> > > > > > > > > +	kvm_set_irq(irqfd->kvm, irqfd->source->id, irqfd->gsi, 1);
> > > > > > > > > +}
> > > > > > > > > +
> > > > > > > > >  /*
> > > > > > > > >   * Race-free decouple logic (ordering is critical)
> > > > > > > > >   */
> > > > > > > > 
> > > > > > > > 
> > > > > > > > Why is it safe to ignore return value here?
> > > > > > > > needs a comment.
> > > > > > > 
> > > > > > > Well, it seems like you and Gleb came to the conclusion that it's safe,
> > > > > > > but I can really follow from the list thread.  Can you explain and I'll
> > > > > > > add a comment?  Thanks,
> > > > > > > 
> > > > > > > Alex
> > > > > > 
> > > > > > We merely talked about edge interrupts.
> > > > > > 
> > > > > In fact it would have been nice to return -EBUSY when write() to level
> > > > > irqfd is coalesced.
> > > > 
> > > > Possibly nice but not really practical.
> > > > 
> > > What do you mean by that? Impossible to implement or not useful?
> > 
> > Impossible to implement and also does not match normal eventfd
> > semantics.
> > 
> Hmm, I remember we discussed using irqfd for level triggered interrupt ~2
> years ago and came to a conclusion that eventfd is a bad fit for it,
> was true than is true now. Not be able to detect coalescing will make
> irqfd level interrupts inferior to IRQ_LINE ioctl.

This aspect isn't different from MSI though, is it?

> --
> 			Gleb.
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