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

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Message-ID: <1340832499.1207.255.camel@bling.home>
Date:	Wed, 27 Jun 2012 15:28:19 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	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, 2012-06-28 at 00:14 +0300, Michael S. Tsirkin wrote:
> On Wed, Jun 27, 2012 at 02:59:09PM -0600, Alex Williamson wrote:
> > On Wed, 2012-06-27 at 12:51 +0300, Michael S. Tsirkin wrote:
> > > On Tue, Jun 26, 2012 at 11:09:46PM -0600, Alex Williamson wrote:
> > > > In order to inject an interrupt from an external source using an
> > > > irqfd, we need to allocate a new irq_source_id.  This allows us to
> > > > assert and (later) de-assert an interrupt line independently from
> > > > users of KVM_IRQ_LINE and avoid lost interrupts.
> > > > 
> > > > We also add what may appear like a bit of excessive infrastructure
> > > > around an object for storing this irq_source_id.  However, notice
> > > > that we only provide a way to assert the interrupt here.  A follow-on
> > > > interface will make use of the same irq_source_id to allow de-assert.
> > > > 
> > > > Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> > > > ---
> > > > 
> > > >  Documentation/virtual/kvm/api.txt |    5 ++
> > > >  arch/x86/kvm/x86.c                |    1 
> > > >  include/linux/kvm.h               |    3 +
> > > >  virt/kvm/eventfd.c                |   95 +++++++++++++++++++++++++++++++++++--
> > > >  4 files changed, 99 insertions(+), 5 deletions(-)
> > > > 
> > > > diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> > > > index ea9edce..b216709 100644
> > > > --- a/Documentation/virtual/kvm/api.txt
> > > > +++ b/Documentation/virtual/kvm/api.txt
> > > > @@ -1981,6 +1981,11 @@ the guest using the specified gsi pin.  The irqfd is removed using
> > > >  the KVM_IRQFD_FLAG_DEASSIGN flag, specifying both kvm_irqfd.fd
> > > >  and kvm_irqfd.gsi.
> > > >  
> > > > +With KVM_IRQFD_FLAG_LEVEL KVM_IRQFD allocates a new IRQ source ID for
> > > > +the requested irqfd.  This is necessary to share level triggered
> > > > +interrupts with those injected through KVM_IRQ_LINE.  IRQFDs created
> > > > +with KVM_IRQFD_FLAG_LEVEL must also set this flag when de-assiging.
> > > > +KVM_IRQFD_FLAG_LEVEL support is indicated by KVM_CAP_IRQFD_LEVEL.
> > > 
> > > Note that if my patch removing auto-deassert gets accepted,
> > > this is not needed at all: we can just look at the GSI
> > > to see if it's level or edge.
> > 
> > I'm not sure this is a good idea.  I know from vfio that I'm injecting a
> > level interrupt regardless of how the guest has the pic/ioapic
> > programmed at the time I'm calling this ioctl.  Peeking across address
> > spaces to get to the right pin on the right pic/ioapic and see how it's
> > currently programmed seems fragile.  Thanks,
> > 
> > Alex
> 
> Fragile? If you set eventfd as LEVEL but GSI is really edge then
> it all explodes, right? So why give users the option to shoot
> themselves in the foot?

If the guest has the ioapic rte set to edge at the time I call KVM_IRQFD
to register my level interrupt then it all explodes, right?  I'd rather
let the user shoot themselves than play Russian roulette with the guest.
Am I misunderstanding what you mean by looking that the GSI to see if
it's level or edge?

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