Re: [PATCH v3 5/5] KVM: Allow host IRQ sharing for passed-through PCI 2.3 devices

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 2010-11-03 at 10:43 +0200, Michael S. Tsirkin wrote:
> On Wed, Nov 03, 2010 at 09:11:16AM +0100, Jan Kiszka wrote:
> > From: Jan Kiszka <jan.kiszka@siemens.com>
> > 
> > PCI 2.3 allows to generically disable IRQ sources at device level. This
> > enables us to share IRQs of such devices between on the host side when
> > passing them to a guest. This feature is optional, user space has to
> > request it explicitly.
> > 
> > Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> 
> 
> I just realized something.
> With this patch, if guest ever looks at
> interrupt disable bit, it will go crazy as that bit goes on/off by
> itself. I guess we could have an ioctl to set/clear the bit on
> device, and have qemu call that on config write into command/status
> register.
> 
> There's also something I don't completely unerstand with current code:
> how does interrupt sharing work? E.g. can assigned and emulated
> devices share an interrupt?

I've been pondering this with VFIO too.  There it seems to work, even
when I enable irqfd.  The VFIO kernel/qemu driver needs to filter EOIs
based on whether the interrupt was actually asserted by the device, but
I think we're likely relying somewhat on interrupts being reasserted to
help us keep everything serviced.

Alex