Interrupt Affinity Filter -- How it works:
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IntFiltr works by intercepting a specific PnP IRP that is passed to 
devices (IRP_MN_START_DEVICE).  

The filter looks at the (translated) resource list that is passed along 
with this IRP, and modifies the affinity mask (according to settings 
specified by the user) for any of the Interrupt resources in the device's 
resource list.  This modified IRP is then passed-on to lower drivers in 
the stack.  

As a result, lower drivers (which use entries in this resource list when 
setting up their ISRs) will get their ISRs installed only on the set of 
processors that the user has specified (via the IntFiltr configuration 
tool).  

The end result is that the APIC gets programmed to direct certain 
interrupts to certain subsets of the CPUs in the system (which can be 
verified by doing a "!ioapic" in the debugger).  

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NOTE: one of the main challenges in writing this filter was ensuring 
that it could be installed on _any_ device stack.


