NetBSD Problem Report #56399

From www@netbsd.org  Tue Sep 14 01:44:21 2021
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Date: Tue, 14 Sep 2021 01:44:19 +0000 (UTC)
From: carenas@gmail.com
Reply-To: carenas@gmail.com
To: gnats-bugs@NetBSD.org
Subject: NetBSD HEAD snapshot i386 fails to boot under HyperV (64 bit)
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>Number:         56399
>Category:       kern
>Synopsis:       NetBSD HEAD snapshot i386 fails to boot under HyperV (64 bit)
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Sep 14 01:45:00 +0000 2021
>Originator:     Carlo Arenas
>Release:        any available snapshot of HEAD for i386
>Organization:
>Environment:
>Description:
Trying to boot in a single or multi CPU VM will result in a KDB prompt
after failing to initialize the CPUs as shown by :

...
[  1.0000030] vmbus0 at acpi0 (VMB8, VMBUS-0): Hyper-V VMBus
[  1.0000030] fatal protection fault in supervisor mode
[  1.0000030] trap type 4 code 0 eip 0xc0488dab cs 0x8 eflags 0x246 cr2 0 ilevel 0x8 esp 0x4
[  1.0000030] curlwp 0xc14e8f00 pid 0 lid 0 lowest kstack 0xc17902c0
kernel: supervisor trap protection fault, code=0
Stopped in pid 0.0 (system) at netbsd:vmbus_init_synic_md+0xa6:    wrmsr

NetBSD 9.2 (release or current) doesn't show the problem, and neither does an AMD64 snapshot in the same hypervisor.
>How-To-Repeat:
Under Hyper-V (using it locally in Windows 10, but should also behave the same in Azure), create a new VM machine or upgrade a running VM from NetBSD 9.2 and try to boot.
>Fix:

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