> I need to verify what we are seeing though -- is it a hardware FIFO
> overrun, or the ring buffer filling up?  com(4) shows hardware fifo
> overflows as "silo overflows" and ring buffer fills as "ibuf floods".

I see

	com1: 1 silo overflow, 0 ibuf floods

from time to time on a full-time modem link with a 350MHz P4.  The serial
port is a 16550A (or emulation thereof) with a putatively "working fifo",
according to dmesg.

This is probably something simply sitting on the interrupt mask for way
too long, and the com(4) driver is thus not at fault.  But it would be nice
to figure out what is blocking interrupts for tens of milliseconds or however
long this works out to be...