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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Nov 03, 2010 at 09:51:10AM +0100, Jan Kiszka wrote:
> Am 03.11.2010 09:43, 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.
> 
> I understand the problem, but I don't get why the kernel should bother.
> User space has to filter the config space access, returning precisely
> the value of the INTx disabled bit that the guest wrote.

Yes but if guest disables INTx it should not get interrupts :)

> > 
> > 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?
> 
> You mean on the guest IRQ line (host-side sharing is obviously fine)?
> Don't know, but that wouldn't be a new issue. Need to study the sources
> /wrt IRQ line arbitration and concurrent use.
> 
> Jan
>