Re: [Qemu-devel] [Bug 1819289] Re: Windows 95 and Windows 98 will not install or run

From: Maxim Levitsky <mlevitsk@redhat.com>
To: Aleksandar Markovic <aleksandar.m.mail@gmail.com>,
	Bug 1819289 <1819289@bugs.launchpad.net>
Cc: qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [Bug 1819289] Re: Windows 95 and Windows 98 will not install or run
Date: Wed, 21 Aug 2019 16:59:32 +0300	[thread overview]
Message-ID: <51d9225a35fa87339a2045029a05f4dda8bfb837.camel@redhat.com> (raw)
In-Reply-To: <CAL1e-=gp6ZEP8GeseG9hPJbP7=ma4+GkV7RLrV8ttqQvnUQJxA@mail.gmail.com>

On Wed, 2019-08-21 at 00:31 +0200, Aleksandar Markovic wrote:
> 20.08.2019. 23.56, "Brad Parker" <1819289@bugs.launchpad.net> је написао/ла:
> > 
> > So it looks like even though that commit fixed it, it seems to break
> > again (differently) in 3.0.0, so I'll need to do another bisect between
> > cfcca36 and v3.0.0 then I guess. And keep working my way up to master as
> > well.
> > 
> > --
> 
> At least you will have an interesting debugging story to tell afterwards. I
> wish you luck!

Does disabling the boot logo help? I do have a working win98 VM here 
(nostaligia reasons, that was my first and last primary OS prior to me
switching to Linux) and what you describe seems to be the hang during the
win98 boot logo?

If it is disabled (logo=0 in msdos.sys, under [Options])
or if you quickly press escape while the logo just showed up, the vm won't hang,
and work normally.

The funniest thing is that when _KVM_ is disabled (that is TCG is used instead), the bug doesn't happen,
so it might instruction emulation bug in KVM. Or something timing related.

I also usually see even a message from the in-kernel instruction decoder about
'cmpchg decoded as nop' or something like that, but don't seem to see it anymore on current kernel.

I just verified these claims with qemu-4.1.0, kernel 5.3.0-rc4, and an win98 (although I only booted it with and without logo few times,
and didn't do an install)

Speaking of other issue you will face with win98 is that it doesn't detect the PnP bios by default, so it doesn't see any devices qemu exposes
on the PCI bus. Thankfully this can be fixed by explicitly overriding the driver
http://computernewb.com/wiki/Install_QEMU_Tablet_Driver_on_Windows_98

For other hardware I had luck with pcnet network card, AC97 sound (SB16 works too but it tends to block mouse during the sound,
probably some PIO/interrupt whatever stuff that is very emulation heavy. For AC97 it was really hard to find a working driver,
but eventually I found some Win95 old style driver (not the newer WDM drivers that just don't work).

It was really fun weekend, few months ago when I set the thing up :-)

Best regards,
	Maxim Levitsky