RE: [Xen-devel] real-mode switch

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

>>> "Jayant Mangalampalli" <Jayant_Mangalampalli@xxxxxxxxxxx> 06.06.07 07:16 >>>
>Can you please explain why you think it is dangerous? Especially if it
>is supported by functionality to switch to protected mode right after
>"real mode code" is done executing?

I'm not sure I understand the second part of your statement - by the time
dom0 gets control, real mode execution has long stopped (in fact, in pure
-unstable Xen there's not a single bit of real mode execution at present,
though there are patches to switch back to real mode very early during
Xen startup to retrieve EDD and EDID info and in the future to establish
a user selected video mode, but those haven't been accepted so far).

The problems with real mode code execution after initial boot is that once
interrupts and certain types of exceptions got enabled, they would need
to be masked over an unbounded time period, risking either losing of them
or failing to handle them in time. On SMP systems, this would include either
halting all remote CPUs or gracefully dealing with IPIs that may be sent
against the CPU switching to or being in real mode (again over an
unbounded period of time).

Jan


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