i/o is very slow on FreeBSD dom0 with Xen-4.12 and Freebsd-12

[Brian Buhrow]

	hello.  Another data point on this issue.  It appears the problem may
be with the onboard IDE controllers, or, possibly, the way interrupts are
handled.  If I switch the BIOS to enable ahcisata mode instead of IDE mode
for the SATA controllers, things boot more quickly, but the FreeBSD kernel
is unable to talk to the hard drives.  The ahci channels time out and we
get stuck in an endless loop of CAM system retries.  
	When the BIOS is set to Enhanced IDE mode, the FreeBSD kernel can talk
to the drives, but the data rate is so slow as to be unusable.  
	When FreeBSD boots on bare metal it can talk to the ahci or IDE drives
without a problem.  This makes me think Xen is messing with the interrupt
handling of the IDE controller somehow.  Is it possible the BIOS sets
things up in a way that FreeBSD likes, but Xen does not?  Perhaps the
ioapic is programmed in a way that FreeBSD doesn't like?
-thanks
-Brian