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

[Jan Kiszka <jan.kiszka@web.de>]

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Jamie Lokier wrote:
> 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.

I checked some old documents: It's a combination of both. Some CMOS byte 
(0x0f) signals the special reset, and a pointer in the BIOS memory 
(0x40:0x67) describes the desired jump target.

> 
>> 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.

Yeah, meanwhile my brain seems to work again and actually read what you 
mean.

But I'm still not convinced that we should make a special case about 
this in the QEMU core. The user is not forced to enable reset logging, 
and maybe (s)he _does_ want to log also resets due to protected mode 
exits - what then? Keep it simple, just log what actually happens if 
logging is enabled.

> 
> 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 :-)

Understanding the usage of setjmp/longjmp in QEMU is a key to grasp the 
control flow - but it took me some time as well to realize this. :)

Jan


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