Re: [Xen-devel] [PATCH v5 2/4] shutdown: Prepare for use of an enum in reset/shutdown_request

[Eric Blake <eblake@xxxxxxxxxx>]

On 04/28/2017 11:09 AM, Dr. David Alan Gilbert wrote:

>>> At a higher level, using your tags, I'm not sure where a reset triggered
>>> by a fault detected by the hypervisor lives - e.g. an x86 triple fault
>>> where the guest screws up so badly that it just gets reset.  Is
>>> that a guest-reset or a guest-panic or what - neither case
>>> was actually asked for by the guest itself.
>>
>> Wouldn't that be host-error (qemu detected an error that prevents
>> further execution of the guest without a reset - and a triple fault
>> seems to fall into the category of the guest getting itself wedged
>> rather than actually trying to reset)?  Except patch 3 only used
>> SHUTDOWN_TYPE_HOST_ERROR in the xen portion of the patch.
>>
>> So if any x86 expert has an opinion on where triple-fault handling is
>> emulated, and what category should be used there, I'm welcome to
>> tweaking this series.
> 
> It's pretty much on the border anyway, I don't think it matters too
> much; it sounds perfectly reasonable.

Actually, reading
https://blogs.msdn.microsoft.com/larryosterman/2005/02/08/faster-syscall-trap-redux/
makes it sound like the triple-fault = reset is exploited by existing OS
(dating back to days of targetting 286 machines), so it is bare-metal
behavior that we have to faithfully emulate as a guest-triggered reset,
and not something where the guest has wedged itself to the point where
qemu can no longer execute the guest.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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