Re: [Qemu-devel] Re: [RESENT][PATCH 2/2] x86: Issue reset on triple faults

[Natalia Portillo <claunia@claunia.com>]

A CMOS byte?

Is there really a way to distinguish between wanted 
reset-by-triple-fault or a non-wanted-triple-fault?

As far as I know the first guest that used this for context switching 
(OS/2 1.x) used it as an undocumented but convenient bug/feature of 286 cpu.

Jamie Lokier escribió:
> Jan Kiszka wrote:
>   
>>> It might be worth distinguishing between
>>> triple-fault-used-by-guest-for-context-switch and triple faults which
>>> trigger a normal reset, and log only the latter.  There's a
>>> standardish way of distinguishing them, which the BIOS should check.
>>>       
>> You refer to setting some return address at some magic BIOS location?
>>     
>
> Probably; I forget the details.  There might be a CMOS byte, too.
>
>   
>> Isn't this something the BIOS should handle, not QEMU?
>>     
>
> The BIOS should handle it, yes.  But since it is standard behaviour,
> it might be useful for QEMU to decide whether to _log_ the event as a
> system reset based on that state.
>
> Same for keyboard controller induced reset - that's also used for
> context switching, in the same way.  (Triple fault is only used
> because it's faster.)
>
> Same also for deciding whether -no-reboot should close down the QEMU
> process.  Now I think about it, that is the best reason to distinguish
> them!
>
>   
>>> When helper(SVM_EXIT_SHUTDOWN, 0) is called, should it still also call
>>> qemu_system_reset_request()?
>>>       
>> helper_vmexit() is not expected to return (cpu_loop_exit).
>>     
>
> Ok.  It's not clear unless you know the code, which I don't.  Just
> wanted to check :-)
>
> -- Jamie
>
>
>
>