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

["Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx>]

* Eric Blake (eblake@xxxxxxxxxx) wrote:
> 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.

The point is it's both :-)
A lot of x86 reset code tries four or five different ways to invoke
a reset and if all else fails they triple fault.

Dave

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



--
Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK

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