Re: Coupling between KVM_IRQFD and KVM_SET_GSI_ROUTING?

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On Mon, Jun 23, 2014 at 06:47:51PM +0200, Alexander Graf wrote:
> 
> On 17.06.14 13:39, Eric Auger wrote:
> >Hello,
> >
> >I have a question related to KVM_IRQFD and KVM_SET_GSI_ROUTING ioctl
> >relationship.
> >
> >When reading the KVM API documentation I do not understand there is any
> >dependency between KVM_IRQFD and KVM_SET_GSI_ROUTING. According to the
> >text it seems only the gsi field is used and interpreted as the irqchip pin.
> >
> >However irqchip.c kvm_set_irq code relies on an existing and not dummy
> >routing table.
> >
> >My question is: does anyone agree on the fact the user-side must set a
> >consistent routing table using KVM_SET_GSI_ROUTING before using
> >KVM_IRQFD? The other alternative would have been to build a default
> >identity GSI routing table in the kernel (gsi = irqchip.pin).
> 
> I untangled irqfd support from the x86 ioapic emulation a while back. When I
> looked at it, I didn't see any easy way to split it out from the routing
> too, so I kept that dependency in.
> 
> If you look at the code, you will see that the irq routing entry is used as
> token for an irqfd. So every irqfd only knows its routing table entry,
> nothing else.

As far as I can see, the routing table entry is used for only two
things: to tell whether the interrupt is an MSI or LSI, and to pass to
kvm_set_msi.

One way to tackle the problem would be to abstract out the routing
table lookup into a function in irqchip.c, rather than having it
open-coded in irqfd_update.  Then, if you don't want to have the
routing table you write an alternative function that creates a
routing entry from scratch.  It would need information from the
low-level irq chip code as to which interrupts are LSIs and which are
MSIs.  It also ties you down to having only one kind of interrupt
controller.

The other way I see to solve my particular problem, which is that
userspace tends to use hardware interrupt numbers starting at 4096, is
to replace the arrays in struct kvm_irq_routing_table with idr
structures or something similar.

> Splitting that dependency out is certainly quite tedious work. However, I'm
> sure the IBM folks will be grateful if you do it :).

Indeed. :)

Paul.
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