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

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Date:	Wed, 27 Jun 2012 14:50:16 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	"Michael S. Tsirkin" <mst@...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 02:38:43PM +0300, Michael S. Tsirkin wrote:
> On Wed, Jun 27, 2012 at 01:22:54PM +0300, Gleb Natapov wrote:
> > On Wed, Jun 27, 2012 at 01:20:24PM +0300, Michael S. Tsirkin wrote:
> > > 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.
> > If we will start dropping everything that was never documented we will
> > hardly have working VMM at the end :)
> 
> Generally true.
> 
> However I just realized that this interface does not work
> reliably for ioapic/pic edge either. More or less for the same reason that
> it does not work for edge: we set bit in IRR, try to inject,
> then clear even if injection failed (e.g. because it is low
> priority). Guest will never get an interrupt in this case.
> 
That's not how ioapic/pic edge interrupts work.

> So what happened e.g. for Alex's vfio prototype is he never
> hit an interrupt window in light testing.
> 
> What we would need to do to fix it is:
> > > > > -	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 1);
> > > > > -	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 0);
> > > > > +	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 0);
> > > > > +	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 1);
> 
> Which will fix edge but break level same as this patch.
> But is it worth fixing? Maybe declare it broken and be
> done with it?
> 
> 
> > > 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.
> > 
> > --
> > 			Gleb.

--
			Gleb.
--
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