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

[Kevin Wolf]

Jamie Lokier schrieb:
> Kevin Wolf wrote:
>>> IMHO There's no reason to print a message to stderr. This is all
>>> well defined behavior, and the accepted way of exiting from 286
>>> protected mode.
>> How many users does qemu have who need triple faults to exit from 286
>> Protected Mode?
> 
> 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.

>> And how many users does it have who don't use triple
>> faults (yes, it's called a fault, not a PM exiting feature)
> 
> Faults don't mean errors.  Think about page faults.

Ok, you're right on my wording. But double faults (which always occur
before a triple fault) are in fact despite their name not faults, but
aborts. Just to reword it using the correct terminology. ;-)

>> And honestly, a message on stderr really shouldn't hurt those 286 folks.
> 
> 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.

Kevin