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

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

Looks like this is the most vigorous technical discussion
we've had on this list in a long time!  Unfortunately I
have to admit I haven't been able to keep up with all
of the details, but I do have a couple of comments:

1) Interrupts may happen at a rate of tens of thousands
   per second.  Just like all high frequency CPU operations
   are coded with "fast paths" (hyperprivops and hyperreflection),
   I think interrupt reflection (some call it injection)
   needs to be implemented with a fast path.  Unlike the
   CPU ops, there is currently no fast path for external
   interrupt reflection, though many of the CPU ops that
   a guest performs (e.g. ivr, eoi, tpr) DO have fast paths.

   When an external interrupt arrives (e.g. Xen is executing
   starting at IVT+3000), the vast majority of interrupts
   should be able to be reflected or recorded using a fast
   path.  This is much harder to do with event channels than
   by setting a bit in a hyperregister.  (Sure you could
   rewrite all the event channel code in assembly, but
   then what is the point of sharing the C code?)

2) Eddie commented that all the event channel code is already
   used in Xenlinux/ia64.  Not true.  There is a separate
   file (evtchn_ia64.c) that is used instead.

3) I don't think we should be trying to support machines and
   configurations that Linux is not even yet able to support
   adequately.  We have plenty of work to do to get Xen/ia64
   usable.  And sharing IRQs between driver domains may be
   necessary eventually, but it doesn't seem a huge restriction
   in the short term to not allow different driver domains to
   share the same IRQ.

Dan

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