What are managed IRQs?

Hi everyone,

isolcpus has the option `managed_irq`, which supposedly assigns the IRQs to
housekeeping CPUs if possible. According to the description of isolcpus, the
affinity of managed IRQs can not be changed via /proc/irq.
I've searched the kernel documentation and the kernel source code for 
more information about managed IRQs / managed interrupts, but haven't found 
any documentation (aside from isolcpus).

Specifically, I'm interested in what makes managed IRQs different (as in 
why can't the affinity be changed via procfs), and
what kind of IRQs are "managed"? 

Also, I'd like to verify if the cmdline parameter `irqaffinity` 
affects managed IRQs, or if it only modifies the default affinity for IRQs
which can be changed via procfs.

Thanks

Jonathan