floppy not working in Linux

["Jan Beulich" <JBeulich@novell.com>]

We had a report that the floppy driver isn't working with current Xen.
While certainly not a major concern to most of us, it still seems odd that
this simple a piece of hard-/software wouldn't work under Xen, especially
as it is known that it had been working a couple of years back.

Trying to debug the issue has surfaced quite a perhaps strange mix of
observations:

- On an old (32-bit) box of mine I found that the driver would work
on Xen 3.1, while the same kernel running on top of 3.2.x would not
work (nor would a reasonably current kernel on a reasonably current
Xen).
- On both this old and not too old a 64-bit box I found that removing
the REALLY_SLOW_IO definition at the top of floppy.c would reduce
the amount of errors encountered quite significantly. Specifically, I
have never seen a mount fail with that change in place (but there
are occasional errors logged by the driver).
- Generally, the problems start with unexpected interrupts or, less
frequently, "-- FDC reply error" messages.

While the driver certainly didn't change much over the last couple
of years, the effect of the adjustment above suggests that the
driver itself may have problems which are just unnoticeable when
run on bare hardware.

Given the timing requirements to service the FDC (a maximum of
about 120us when using the FIFO optimally) I wonder whether it
is reasonable to expect this to work reliably at all, especially
when guest domains are consuming non-neglectable amounts of
CPU time.

Jan