install/55955: guest NetBSD 9.1 does not boot in qemu-kvm with "-cpu host" on Phenom II X4 970 CPU on ubuntu 20.04

[lvd.mhm%gmail.com@localhost]

>Number:         55955
>Category:       install
>Synopsis:       guest NetBSD 9.1 does not boot in qemu-kvm with "-cpu host" on Phenom II X4 970 CPU on ubuntu 20.04
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    install-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Jan 24 13:55:00 +0000 2021
>Originator:     Vadim
>Release:        9.1
>Organization:
>Environment:
host OS: linux ubuntu 20.04
host uname -a: Linux **** 5.4.0-62-generic #70-Ubuntu SMP Tue Jan 12 12:45:47 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
qemu: 5.2.0 (compiled from sources) or 4.2.1 (from ubuntu-supplied packages)
>Description:
When I start qemu 5.2.0 like:

./qemu-system-x86_64 -accel kvm -hda NetBSD-9.1-i386-install.img -cpu host

Boot process does not go behind a very first string with 6 numbers, rebooting the guest machine quietly and immediately.

This no-boot happens also with "-cpu Phenom" and "-cpu Opteron_G3" but not with other "Opteron_Gn", n=1,2,4,5 (they do boot).

This also happens with other install images: NetBSD-9.1-amd64-install.img, NetBSD-9.1-i386.iso and NetBSD-9.1-amd64.iso.

Exactly same also happens when using "qemu-system-i386", obviously only for "i386" images.

The same no-boot happened with 9.0 install images, although I've not tested them thoroughfully.

When I try to boot my host machine off an USB stick with NetBSD-9.1-i386-install.img on it, it boots (at least) up to the keyboard selection menu.

NetBSD-8.2-amd64.iso install image seems to boot OK in qemu under the same circumstances.

>How-To-Repeat:
1. get host machine with the mentioned CPU and ubuntu 20.04 on it

2. get and compile qemu-5.2.0 with KVM support. (although qemu-4.2.1 from ubuntu packages also shows that bug)

3. run NetBSD install images as described.

>Fix:
Do not use "-cpu host" or "-cpu Phenom" or "-cpu Opteron_G3" option, which however does not allow guest OS to see host CPUID flags.

"-cpu host" is the default option in ubuntu-provided "virt manager" virtualization software.