RE: [Xen-devel] real-mode switch

["Jayant Mangalampalli" <Jayant_Mangalampalli@xxxxxxxxxxx>]

I understand ... but wouldn't it be helpful if we can have the ability
to switch to real mode to execute a BIOS call and then come back to
protected mode as soon as we are done. This feature could potentially be
misused, but it could be real useful when we want to quickly update
something through bios.

Thanks
Jayant

-----Original Message-----
From: Jan Beulich [mailto:jbeulich@xxxxxxxxxx] 
Sent: Wednesday, June 06, 2007 12:39 PM
To: Jayant Mangalampalli
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-devel] real-mode switch

>>> "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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