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port-xen/56328: XEN3_DOM0 bcount <= MAXPHYS panic...

>Number:         56328
>Category:       port-xen
>Synopsis:       XEN3 DOM0 panic with bcount <= MAXPHYS in xbdback_xenbus.c
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    port-xen-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Jul 24 22:55:00 +0000 2021
>Originator:     Greg Oster
>Release:        NetBSD 9.99.87
System: NetBSD merlin 9.99.87 NetBSD 9.99.87 (XEN3_DOM0) #0: Fri Jul 23 22:39:05 CST 2021  oster@gonzo:/u1/builds/build350/src/obj/amd64/u1/builds/build350/src/sys/arch/amd64/compile/XEN3_DOM0 amd64

Architecture: x86_64
Machine: amd64

When a Ubuntu 20.04.2 DOMU (with a 4.6.7 Linux kernel) is running on a 
NetBSD 9.99.87 DOM0, the following panic is encountered:

[50.8700852] panic: kernel diagnostic assertion "bcount <= MAXPHYS" failed: 
file "/u1/builds/build358/src/sys/arch/xen/xen/xbdback_xenbus.c", line 1320
[50.8700852] cpu0: Begin traceback...
[50.8700852] vpanic() at netbsd:vpanic+0x14a
[50.8700852] kern_assert at netbsd:kern_assert+0x4b
[50.8700852] xbdback_co_io_gotio() at netbsd:xbdback_co_io_gotio+0x202
[50.8700852] xbdback_thread() at netbsd:xbdback_thread+0x9b
[50.8700852] cpu0: End traceback...

when the domain is started.


Upgrade a DOM0 kernel from NetBSD 9.2 to 9.99.87.  Watch the machine panic 
after starting DOMUs.  Hunt for a fix, and discover that bcount is 
MAXPHYS+PAGE_SIZE (i.e. 69632), and that there are 16 segments in the IO
that is failing.  Wonder how a DOMU could have issued an IO larger than
MAXPHYS, and then realize the Ubuntu DOMU might be doing that.  Test booting
without the Ubuntu DOMU, and it works fine.  Add Ubuntu DOMU back, and it fails. 

Ubuntu DOMU is:
Welcome to Ubuntu 20.04.2 LTS (GNU/Linux 4.6.7-040607-generic x86_64)

Please. :)
(I'm happy to help test any proposed fix, as it's easy to replicate the issue
on my Xen machine.)

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