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

["Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>]

>    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).

Then the second question is: even if shared IRQ
support must be implemented in Xen/ia64, is there
really a need for two different domains to share
an IRQ?  I'd imagine the need for this would be
very close to zero.

Then the third question:  Is there a performance
cost for shared IRQ support, even on systems that
don't share IRQ's?  Is there an additional
performance cost for sharing an IRQ across domains?
With each NIC card capable of generating thousands
of interrupts/second, it doesn't seem wise to
add additional overhead to every interrupt on
every Xen system to support the possibility that
someone may want to configure a system to share
IRQs across domains.

My two cents,
Dan


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