There are two ways to express an interruption subclass:
- As a bitmask, as used in cr6.
- As a number, as used in the I/O interruption word.
Unfortunately, we have treated the I/O interruption word as if it
contained the bitmask as well, which went unnoticed so far as
- (not-yet-released) qemu made the same mistake, and
- Linux guest kernels don't check the isc value in the I/O interruption
word for subchannel interrupts.
Make sure that we treat the I/O interruption word correctly.