kern/55607: Panic 9.0_STABLE Xen PV with ZFS module

[brad%anduin.eldar.org@localhost]

>Number:         55607
>Category:       kern
>Synopsis:       Panic 9.0_STABLE Xen PV with ZFS module
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Aug 25 20:35:00 +0000 2020
>Originator:     brad%anduin.eldar.org@localhost
>Release:        NetBSD 9.0_STABLE
>Organization:
	eldar.org
>Environment:
System: NetBSD samwise.nat.eldar.org 9.0_STABLE NetBSD 9.0_STABLE (XEN3_DOMU) #0: Sat May  9 22:12:48 EDT 2020  brad%samwise.nat.eldar.org@localhost:/lhome/NetBSD_9_branch_20200509/amd64/OBJ/sys/arch/amd64/compile/XEN3_DOMU amd64
Architecture: x86_64
Machine: amd64
>Description:

I have been using ZFS with a 9.0_STABLE Xen PV system for some time
with pretty good results.  The last time that the system was updated
was 2020-05-09 and all was ok.  I went to update the system to
2020-08-20 and when the zfs module loads the DOMU guest panics with
this:

[   3.8500388] fatal page fault in supervisor mode
[   3.8500388] trap type 6 code 0x2 rip 0xffffffff8022abba cs 0xe030 rflags 0x10202 cr2 0xca000000eb ilevel 0x8 rsp 0xffff8c00d2063c30
[   3.8500388] curlwp 0xffff8c0007460040 pid 0.2 lowest kstack 0xffff8c00d20602c0
kernel: page fault trap, code=0
Stopped in pid 0.2 (system) at  netbsd:xbd_handler+0x223:       movl    $0x5,20(
%r13)
xbd_handler() at netbsd:xbd_handler+0x223
xen_intr_biglock_wrapper() at netbsd:xen_intr_biglock_wrapper+0x1d
evtchn_do_event() at netbsd:evtchn_do_event+0xe8
do_hypervisor_callback() at netbsd:do_hypervisor_callback+0x161
Xhypervisor_pvhvm_callback() at netbsd:Xhypervisor_pvhvm_callback+0xa0
idle_loop() at netbsd:idle_loop+0xb6
ds          16
es          b910
fs          0
gs          d3
rdi         6
rsi         ffff8c00d3556000
rbp         ffff8c00d2063ca0
rbx         1d
rdx         9a0
rcx         16
rax         ffff8c000846b910
r8          698
r9          32
r10         0
r11         246
r12         0
r13         ca000000cb
r14         ffffffffffffffff
r15         ffff8c000846b000
rip         ffffffff8022abba    xbd_handler+0x223
cs          e030
rflags      10202
rsp         ffff8c00d2063c30
ss          e02b
netbsd:xbd_handler+0x223:       movl    $0x5,20(%r13)

The zfs module from 2020-05-09 actually appears to work with the
2020-08-20 DOMU kernel and doesn't panic which may be suggestive of
something.

>How-To-Repeat:

Try using zfs in a very recent 9.0_STABLE

>Fix:

Don't know, except that -current appears to work fine.