linux-kernel - Re: [PATCH v9 1/2] kvm: Use a reserved IRQ source ID for irqfd

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Message-ID: <20120905145153.GA11048@redhat.com>
Date:	Wed, 5 Sep 2012 17:51:53 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Alex Williamson <alex.williamson@...hat.com>, gleb@...hat.com,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 1/2] kvm: Use a reserved IRQ source ID for irqfd

On Wed, Sep 05, 2012 at 05:35:43PM +0300, Avi Kivity wrote:
> On 08/22/2012 03:41 AM, Michael S. Tsirkin wrote:
> >> 
> >> I assumed you were pointing out the level vs edge interaction.  If we
> >> call that a userspace bug, I can just drop this.  Thanks,
> >> 
> >> Alex
> > 
> > level is userspace bug I think :)
> 
> I don't see how it's a bug.  Suppose we have a vfio device that shares a
> gsi with an emulated device.  The emulated device naturally uses
> KVM_IRQ_LINE (it has no need to re-sample on ADN), while vfio naturally
> has to use irqfd.

Absolutely. But vfio needs to use irqfd with the new flag.
Using existing irqfd for level is a bug.

> Note one would expect that each irqfd gets its own irq source id, since
> they are all independent level sources.  The reason they don't is that
> we shut them down anyway and let the sources re-trigger (it is more
> accurate to say that they have no irq source id, but that would just
> muddle the implementation).
> 
> Alex, if the conclusion is that we do need this patch, then please add a
> comment explaining why we can share the source id among all irqfd users.
> 
> -- 
> error compiling committee.c: too many arguments to function
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