linux-kernel - Re: [PATCH] kvm: don't clear level from irqfd

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Message-ID: <20120627102024.GA17922@redhat.com>
Date:	Wed, 27 Jun 2012 13:20:24 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH] kvm: don't clear level from irqfd

On Wed, Jun 27, 2012 at 01:03:46PM +0300, Gleb Natapov wrote:
> On Wed, Jun 27, 2012 at 12:13:43PM +0300, Michael S. Tsirkin wrote:
> > irqfd sets and clears the level immediately.
> > This used to be necessary for MSI interrupts but isn't anymore.
> > 
> > Note! We never officially supported irqfd for level
> > interrupts but it usually seemed to kind of work,
> > and this change will break it. There are no known
> > real users of that feature though - only old vfio prototypes.
> > Removing this line will make it easier to add
> > real support for level irqfds in the future.
> > 
> Did we officially supported irqfd for ioapic/pic edge interrupts?
> Because those will break to.

Hard to say - as Alex points out it was never documented.
But at least they actually worked reliably. Hmm.  One way to address
this is to add kvm_trigger_irq which will trigger edge interrupts
unconditionally.

Not sure it's worth it though.

> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > ---
> >  virt/kvm/eventfd.c | 1 -
> >  1 file changed, 1 deletion(-)
> > 
> > diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> > index f59c1e8..4db6b01 100644
> > --- a/virt/kvm/eventfd.c
> > +++ b/virt/kvm/eventfd.c
> > @@ -68,7 +68,6 @@ irqfd_inject(struct work_struct *work)
> >  	struct kvm *kvm = irqfd->kvm;
> >  
> >  	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 1);
> > -	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 0);
> >  }
> >  
> >  /*
> > -- 
> > MST
> 
> --
> 			Gleb.
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