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RE: [Xen-users] GPLPV but network problem

To: "Nick Couchman" <Nick.Couchman@xxxxxxxxx>, "Steffen Heil" <lists@xxxxxxxxxxxxxxx>
Subject: RE: [Xen-users] GPLPV but network problem
From: "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
Date: Sun, 20 Jul 2008 13:43:16 +1000
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Thread-topic: [Xen-users] GPLPV but network problem
> James, I'm still curious on the question someone else asked about why
> Xen devices show up on the ISA bus?  I know it probably doesn't really
> make a difference, since it is "virtual" hardware, but that just seems
> little strange.  Definitely not a criticism, just curious about it...

The interrupts that those devices use are virtual and are nothing to do
with the PCI interrupt that the xenpci driver uses. They are triggered
by calling an asm 'int x' instruction from the xenpci driver. It would
be nice to be able to use the PCI interrupts, as there are more of them
available and there are performance improvements to be had by using
non-shared interrupts, but when I ask the Windows PnP manager for an
interrupt it appears that I can only ask for one in the range of 1-15,
and for various reasons, 2, 7, 9, and 15 don't work. 2 and 9 because of
the way the ISA interrupt controllers link to each other, and 7 and 15
for reasons that I can't remember right now.

If I ask for an interrupt in the range 16-63, windows won't give me one.
I can't just use a 16-63 interrupt without allocating them as I need the
PnP manager to set up the irq vector properly. This is the problem of
trying to interface with closed source code like Windows - you are stuck
with what you get :)

I think 'ISA' in this case just means a non-APIC sourced interrupt, the
xen devices are on an 'Internal' bus as far as Windows is concerned.


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