win98se floppy fails to boot with isapc machine

QEMU emulator version 4.1.50 (commit 50d69ee0d)

floppy image from:
https://winworldpc.com/download/417d71c2-ae18-c39a-11c3-a4e284a2c3a5

$ qemu-system-i386 -M isapc -fda Windows\ 98\ Second\ Edition\ Boot.img
SeaBIOS (version rel-1.12.1-0...)
Booting from Floppy...
Boot failed: could not read the boot disk

Bisected following note from http://gunkies.org/wiki/I386-softmmu:

the isapc configuration no longer works... So legacy systems must resort to Qemu 0.9.0 or Qemu 0.10.0 

I get:

fd646122418ecefcde228d43821d07da79dd99bb is the first bad commit
commit fd646122418ecefcde228d43821d07da79dd99bb
Author: Anthony Liguori <email address hidden>
Date:   Fri Oct 30 09:06:09 2009 -0500

    Switch pc bios from pc-bios to seabios
    
    SeaBIOS is a port of pc-bios to GCC.  Besides using a more modern tool chain,
    SeaBIOS introduces a number of new features including PMM support, better
    BEV and BCV support, and better PnP support.

Apparently the bisection result from comment #1 comes from a different issue, the VGA display stays blank, can not test the floppy drive.

Bisecting using the pcbios.bin from fd646122418~, the VGA display stops working at:

345c22aa80d1f6ddfe7898f721fd1be3bccb08f1 is the first bad commit
commit 345c22aa80d1f6ddfe7898f721fd1be3bccb08f1
Author: Gerd Hoffmann <email address hidden>
Date:   Fri Dec 18 12:01:12 2009 +0100

    roms: remove option rom packing logic
    
    Now that we load the option roms via fw_cfg, we can stop copying
    them to the 0xc000 -> 0xe000.  The patch does just that.
    
    Also the rom loader gets simplified as all remaining users of the
    rom loader load the bits at a fixed address so the packing and
    aligning logic can go away.

So finally, the commit that introduced this regression is not directly related to QEMU:

0b8f74488e50f98b04e63157f85fde8a13f8d6aa is the first bad commit
commit 0b8f74488e50f98b04e63157f85fde8a13f8d6aa
Author: Gerd Hoffmann <email address hidden>
Date:   Tue Nov 6 12:23:53 2018 +0100

    update seabios to master snapshot
    
    seabios 1.12 release is planned for november.
    
    update seabios to a master branch snapshot so it gets more testing
    and to make the delta smaller when updating to -final during freeze.

And the (SeaBIOS) winner is...

4a6dbcea3e412fe12effa2f812f50dd7eae90955 is the first bad commit
commit 4a6dbcea3e412fe12effa2f812f50dd7eae90955
Author: Nikolay Nikolov <email address hidden>
Date:   Sun Feb 4 17:27:01 2018 +0200

    floppy: Use timer_check() in floppy_wait_irq()
    
    Use timer_check() instead of using floppy_motor_counter in BDA for the
    timeout check in floppy_wait_irq().
    
    The problem with using floppy_motor_counter was that, after it reaches
    0, it immediately stops the floppy motors, which is not what is
    supposed to happen on real hardware. Instead, after a timeout (like in
    the end of every floppy operation, regardless of the result - success,
    timeout or error), the floppy motors must be kept spinning for
    additional 2 seconds (the FLOPPY_MOTOR_TICKS). So, now the
    floppy_motor_counter is initialized to 255 (the max value) in the
    beginning of the floppy operation. For IRQ timeouts, a different
    timeout is used, specified by the new FLOPPY_IRQ_TIMEOUT constant
    (currently set to 5 seconds - a fairly conservative value, but should
    work reliably on most floppies).
    
    After the floppy operation, floppy_drive_pio() resets the
    floppy_motor_counter to 2 seconds (FLOPPY_MOTOR_TICKS).
    
    This is also consistent with what other PC BIOSes do.

I'm not sure if it's related to SeaBIOS because SeaBIOS configuration is stored in QEMU.
So I think CONFIG_TSC_TIMER should be disabled in seabios-128k.config and the 128k variant of SeaBIOS should be used for isapc.

Please see the thread for details: 

  https://lists.gnu.org/archive/html/qemu-devel/2019-08/msg03924.html

If I disable TSC in SeaBIOS (i.e. CONFIG_TSC_TIMER=n) the floppy can't be booted with or without TSC-capable CPU. So, it can't be fixed on QEMU side.

Fun thing, that if I set CONFIG_TSC_TIMER=n and start it with isa-debugcon, it works:
$QEMU -cpu 486 -M isapc,accel=hvf -fda ~/win98-fe-boot-floppy.img -chardev file,id=debugcon,path=seabios.log -device isa-debugcon,iobase=0x402,chardev=debugcon

This one doesn't work:
$QEMU -cpu 486 -M isapc,accel=hvf -fda ~/win98-fe-boot-floppy.img

The ticket should be closed as soon as SeaBIOS gets updated in QEMU. A patch has been posted to SeaBIOS mailing list:
https://<email address hidden>/thread/XPKQNLVWZX55TSLSXZVY5S5DMFYS4CNO/

The commit fixes the issue in master branch: https://git.qemu.org/?p=qemu.git;a=commit;h=de15df5ead400b7c3d0cf21c8164a7686dc81933

The fix is going to be released in 5.1