Panic: HYPERVISOR_mmu_update failed on old NetBSD-5.2 domU

[Brian Buhrow <buhrow%nfbcal.org@localhost>]

	hello. I've ben running NetBSD-5.2 DomU's on various versions of Xen
for a very long time without a problem.  In fact, I have two installations
of Xen-4.12 runing with FreeBSD-12.1 as the Dom0 with 1 runing
Netbsd-5.2/i386 as a domu with no issues, but the other one, using the same
kernel, gives me a panic, reproducible, as shown below.  I see a number of
pr's associated with this panic message, but since I've been using this
configuration successfully for many years, I'm thinking there's something
different about the non-working Xen install that I'm not seeing.  These are
PV kernels, which I thought meant they were less sensitive to changes in
CPU type than the HVM domu's.  Specifically, the working installation is
using an Intel Xeon 5620 processor while the non-working one is using an
Intel I5-2400 CPU.

	maybe someone knows what I am obviously missing.  Any ideas would be
greatly appreciated.  
Dmesg with panic shown below.
-thanks
-Brian

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NetBSD 5.2_STABLE (RBL_DOMU) #0: Tue Nov 12 10:07:14 PST 2019
	buhrow%lothlorien.nfbcal.org@localhost:/usr/src/sys/arch/i386/compile/RBL_DOMU
total memory = 3072 MB
avail memory = 3009 MB
mainbus0 (root)
hypervisor0 at mainbus0: Xen version 4.12.1
vcpu0 at hypervisor0: Intel 686-class, 3092MHz, id 0x206a7
xenbus0 at hypervisor0: Xen Virtual Bus Interface
xencons0 at hypervisor0: Xen Virtual Console Driver
xencons0: using event channel 2
npx0 at hypervisor0: using exception 16
xenbus: can't get state for device/suspend/event-channel (2)
xbd0 at xenbus0 id 768: Xen Virtual Block Device Interface
xbd1 at xenbus0 id 832: Xen Virtual Block Device Interface
xennet0 at xenbus0 id 0: Xen Virtual Network Interface
xennet0: MAC address 00:4e:46:42:42:ce
xennet0: using RX copy mode
xenbus: can't get state for device/suspend/event-channel (2)
ignore shutdown request: 
xenbus: can't get state for device/suspend/event-channel (2)
xenbus: can't get state for device/suspend/event-channel (2)
boot device: xbd0
root on xbd0a dumps on xbd0b
xenbus: can't get state for device/suspend/event-channel (2)
xenbus: can't get state for device/suspend/event-channel (2)
xenbus: can't get state for device/suspend/event-channel (2)
root file system type: ffs
xenbus: can't get state for device/suspend/event-channel (2)
xenbus: can't get state for device/suspend/event-channel (2)
xenbus: can't get state for device/suspend/event-channel (2)
xenbus: can't get state for device/suspend/event-channel (2)
. . . 

Do some work, i.e. fsck the root disk.
Machine always panics with the same 3 entries in the queue.
 . . . 

xpq_flush_queue: 3 entries 
0x00000007a3826fe0: 0x00000007a3828001
0x000000083ce3ad40: 0x00000007a3828001
0x00000007a3cdcd40: 0x00000007a3828001
panic: HYPERVISOR_mmu_update failed