Re: [kvmarm] [RFC PATCH 0/3] KVM: ARM: Get rid of hardcoded VGIC addresses

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Fri, 2012-10-26 at 13:00 +0200, Jan Kiszka wrote:

> And at latest there you will need the IRQ routing infrastructure of KVM.
> It tells KVM which "virtual IRQ" (badly named "GSI") triggers which
> event at which input, e.g. a physical IRQ line at some IRQ controller or
> a specific message at some MSI receiver. You shouldn't try to invent a
> Power wheel here, rather tune the existing one to become more generic.
> We could even try to get rid of that unfortunate GSI name (when leaving
> aliases behind), though that is cosmetic.

Ok, there must be something wrong with me, I still don't understand what
you are talking about.

What "MSI receiver" ? What physical IRQ line are you talking about ? How
is the kernel involved ?

The only cases I can think of are the association between a virtual
interrupt (ie, an interrupt in the guest interrupt number space) and an
in-kernel source for that interrupt, ie, vhost and PCI pass-through
essentially.

Anything else is under qemu control. IE. MSIs or LSIs generated by
emulated devices are just normal interrupt that go through our ioctl to
trigger the in-kernel source with the same number.

I don't see any "routing" happening anywhere in that picture really. The
firmware calls done by the guest to change the target of interrupts
(which CPU/presentation controller to direct a given interrupt to) are
handled entirely in the kernel in platform specific code and update our
internal ICS state.

> > 
> > I might just miss some subtleties here but so far I haven't been able to
> > figure out how to "shoehorn" our stuff in the very x86-centric existing
> > interfaces to the kernel APICs. In fact all that code is in a generic
> > location in kvm but is really x86/ia64 centric and the interfaces seem
> > to be as well.
> 
> That's not true in general, though you surely find a lot of traces and
> still a few concrete x86 bits under virt/kvm.

Well, I haven't found anything in virt/kvm/irq_comm.c that was of any
use to us. Again, I might be sufferring from a major misunderstanding
here but as far as I can tell, the model is totally different. Besides,
that file has a hard coded list of what looks like completely x86
specific mappings between "GSI" and "interrupt numbers" (again I don't
understand completely the distinction and I don't think we have anything
like it on power).

Ben.