port-i386/59829: The installer doesn't boot qemu with -cpu 486

[nikola%radojevic.rs@localhost]

>Number:         59829
>Category:       port-i386
>Synopsis:       The installer doesn't boot qemu with -cpu 486
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-i386-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Dec 10 09:25:00 +0000 2025
>Originator:     Nikola Radojević
>Release:        10.1
>Organization:
>Environment:
The vm host is a linux machine:
Linux nikola-laptop 6.15.8-artix1-1 #1 SMP PREEMPT_DYNAMIC Thu, 24 Jul 2025 20:27:03 +0000 x86_64 GNU/Linux

The qemu version is 10.1.2
>Description:
When trying to run the netbsd installer in a qemu vm, the kernel hangs before ever reaching userspace.

Here is the full boot log with `boot -svx` on -cpu 486: http://0x0.st/KF7u.txt

Here's the same thing for `-cpu pentium`: http://0x0.st/KF7Q.txt

One curious difference between the two is that 486 lacks the apic timer:
`cpu0: apic clock running at 0 Hz`
So it falls back to i8254 which points to potential issues with either the qemu's emulation of i8254 or netbsd's driver, or both.

Here's the backtrace from when the gdb attached to the stuck vm:
0xc0127fe5 in x86_stihlt ()
(gdb) bt
#0  0xc0127fe5 in x86_stihlt ()
#1  0xc04853ff in acpicpu_cstate_idle_enter (sc=0xc244a800, state=1) at /usr/src/sys/dev/acpi/acpi_cpu_cstate.c:764
#2  0xc0c89bac in idle_loop (dummy=0xc2240040) at /usr/src/sys/kern/kern_idle.c:86
#3  0xc0102011 in lwp_trampoline ()

>How-To-Repeat:
Download the latest iso installation media for i386 and run it with the following qemu command:
qemu-system-i386 -cpu 486 -m 512 -hda hdd.qcow2 -cdrom NetBSD-10.1-i386.iso
>Fix: