RE: [Xen-users] Re: Possible to run Xen inside QEMU?

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Petersson, Mats
> Sent: 02 April 2007 13:22
> To: mlmail@xxxxxxxxxx; xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: RE: [Xen-users] Re: Possible to run Xen inside QEMU?
> 
> > Ok, I'll try different emulations later.
> > 
> > Do you have an idea why xen simply reboots/stops without any
> > Error-Message?
> 
> As a general rule, that indicates a "triple-fault", which is when a
> fault in the processor causes a second fault which in turn causes a
> third fault - it would be possible to let the processor recursively
> fault forever, but it's pretty pointless to do, so there is a layered
> approach of "first fault, double-fault, triple-fault". Double-fault is
> supposed to be handled "clean", but of course there are 
> scenarios where
> the processor just hasn't got a chance to "get out of trouble"
> (typically when some global resource like the IDT or GDT has been
> overwritten with garbage). Triple-fault as such will stop the 
> processor
> with a special "pattern" on the pins to indicate that it is a
> "triple-fault stop".
> 
> Since it's pretty pointless to have a system just sit there after a
> triple-fault, the engineers at this time (at the time of the 80286)
> invented a mechanism in the chipset to do a soft-reset at 
> this point, so
> as to restart the machine. A little while later, someone figured out
> that this was a good way to actually restart a PC, so ever 
> since there's
> been code to "zero GDT, cause a trap" (typically, just load GDT with a
> length of zero followed by an INT3 instruction) to restart 
> the machine.
> This was particularly useful when using 286 processor to get out of
> protected mode (as there was no instruction to disable protected mode
> once in there) - something that had to be done quite 
> frequently in early
> OS/2 to make use of "dos-mode" for example. [It's faster to do a
> triple-fault than it is to talk to the keyboard controller to issue a
> reset-signal that way, which is the alternative method].
> 
> QEMU obviously should follow this pattern of "restart with
> triple-fault", otherwise it's "broken" for certain old applications. 

After all that, I just realized that this is bogus: it's not a
triple-fault if "noreboot" stops it from rebooting!

I don't know what would cause an immediate reboot on Xen like that. 

--
Mats
> 
> --
> Mats
> > 
> > 
> > regards
> > 
> > Johannes
> > 
> > 
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