Re: [PATCH v2] x86/vm86: Remove deprecated VM86 support

[Jari Ruusu]

Brian Gerst <brgerst@xxxxxxxxx> wrote:
> Jari Ruusu <jariruusu@xxxxxxxxxxxxxx> wrote:
> > My current setup uses virtualization:
> >
> > 64-bit x86 host + 64-bit Linux kernel/distro + Qemu/KVM
> >   --> boots 32-bit Linux kernel/distro ( X86_LEGACY_VM86=y )
> >     --> boots dosemu + FreeDOS
> >       --> runs DOS version of WordPerfect
> 
> Can you run FreeDOS on Qemu/KVM directly,
> without the 32-bit Linux guest in between?

Currently it is not possible to install dosemu or FreeDOS to
that 64-bit x86 host, at least not from Debian pre-compiled
packages, because Debian dropped dosemu and FreeDOS from
installable pre-compiled packages long time ago.

More information about dosemu+FreeDOS emulated VM86 overhead.
Compiling, assembling, and linking a DOS .exe file from source
using Turbo-C. This is memory+CPU+disk intensive load:

CONFIG_X86_LEGACY_VM86=y        --> 3.1 seconds
CONFIG_X86_LEGACY_VM86 not set  --> 9.4 seconds

Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> On 5/28/26 22:20, Jari Ruusu wrote:
> > With CONFIG_X86_LEGACY_VM86=y set in 32-bit Linux kernel,
> > currently running linux-6.12.91, one memory+CPU intensive
> > operation in DOS version of WordPerfect takes less than 0.5
> > seconds. With CONFIG_X86_LEGACY_VM86 not set, same operation
> > takes about 2 seconds.
> 
> Just out of curiosity, what operation is that? How often do
> you do that operation in a day or a month?

That WordPerfect operation is "re-generate indexes" (page
numbers and such) for 42 page document.

I don't need to do that often at all. Best case: never.

> Just like Brian asked, I'm confused why this needs the
> 32-bit Linux distro in the mix.

That is for historical reasons. Dosemu and FreeDOS were
installed long time ago to some version of Debian on a VM, when
dosemu and FreeDOS were still installable pre-compiled packages
from Debian. That VM was then upgraded in-place from one version
to next, all the way to current Debian-12 version as it is now.
At the time of dosemu and FreeDOS install, it was believed
(correctly or incorrectly, not sure) that dosemu host Linux
kernel must have CONFIG_VM86=y as hard requirement, so 64-bit
x86 host for dosemu was ruled out.

> But, if I were you, I'd just keep the 32-bit distro image
> around, off the Internet, and never update it. Just like the
> version of DOS you have and WordPerfect. The 32-bit distro
> doesn't need a mainline kernel. It just gets frozen in place
> with a 2026 kernel, just like your 1994 version of WordPerfect
> and DOS are frozen.

That is my plan. I will keep updating that 32-bit VM as long as
Debian provides updates for Debian version 12, and then leave
it as it is.

--
Jari Ruusu  4096R/8132F189 12D6 4C3A DCDA 0AA4 27BD  ACDF F073 3C80 8132 F189