Re: [Qemu-devel] Re: [PATCH] Reboot CPU on triple fault

[Jamie Lokier <jamie@shareable.org>]

Kevin Wolf wrote:
> > Not many, only those users running old MS-DOS apps / OSes.
> 
> MS-DOS is running in Real Mode, you'd additionally need an application
> which uses 286 PM. Apparently there aren't too many users relying on
> triple faults as in the past qemu aborted (or in earlier versions even
> hung) on triple faults and nobody complained.

That's true. I think they use Bochs instead.

> > For old MS-DOS apps / OSes, I have the impression this can happen
> > hundreds of times per second.  It's part of task context switching and
> > BIOS calls.
> 
> I have to admit that I'm all but an 286 PM expert. What exactly is a
> triple fault doing there? If it leads to a reset as implemented here, I
> can hardly imagine that it is of much use in regular context switching.

CPU reset is the only way to exit 286 protected mode, and that is
needed when context switching between real and protected mode tasks.

So apps set a byte in the CMOS NVRAM, trigger a processor reset, then
the BIOS reset code immediately jumps back to the app in real mode,
avoiding the normal reboot process.

It's slow and silly, but faster than the other method, asking the
keyboard controller to do it.

-- Jamie