RE: [Xen-ia64-devel] vIOSAPIC and IRQs delivery

[Alex Williamson <alex.williamson@xxxxxx>]

On Wed, 2006-03-08 at 12:00 -0800, Magenheimer, Dan (HP Labs Fort
Collins) wrote:
> >    We agree IOSAPIC must belong to Xen.  And it should be 
> > able to deliver 
> > interrupts to domains and handle shared IRQs.
> 
> Did I miss an answer to Tristan's earlier question,
> which was approximately: How many systems out there
> require shared IRQ's?  I realize there are some huge
> mainframe-class boxes that have hundreds of I/O cards
> that probably do require shared IRQ's, but I wonder
> if this class of machine will even consider using
> Xen?  Mainframe-class machines have other partitioning
> technologies with customer-expected features that Xen
> will never have (such as hardware fault containment).

   Hopefully I'm not stepping into a rat-hole here, but what are we
defining as a shared IRQ?  If we're only talking about running out of
external interrupt vectors on the CPU and programming multiple IOSAPIC
RTEs to trigger the same vector, I agree.  That case requires are rather
large system.  Eventually we should support this, but things like
interrupt domains may be a better long term solutions.

   There's also the case where due to hardware design decisions,
functions within a PCI device or multiple PCI devices may be hard wired
to pull the same interrupt lines.  In this case, multiple devices may
interrupt on the same IOSAPIC RTE.  This is probably pretty rare on ia64
systems today, maybe even limited to multi-function PCI devices.  I
wouldn't expect multiple high speed devices to share an interrupt line
though, so supporting this should have minimal impact on the non-shared
interrupt path.

        Alex

-- 
Alex Williamson                             HP Linux & Open Source Lab


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