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Re: [Xen-devel] Ideas for PV on SeaBIOS

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] Ideas for PV on SeaBIOS
From: Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>
Date: Thu, 19 May 2011 10:17:17 +0200
Cc: Daniel Castro <evil.dani@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, seabios@xxxxxxxxxxx
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On 05/19/11 10:08, Jan Beulich wrote:
On 19.05.11 at 07:33, Daniel Castro<evil.dani@xxxxxxxxx>  wrote:
In order to give PV Drivers to SeaBIOS we will need to solve a few
problems, one is the following:
Does a booting kernel informs the BIOS that it will leave real mode
and not use it again? When the booting kernel uses CPU real mode for
the last time, how can we (Xen or SeaBIOS) know that real mode will no
longer be used, and hence BIOS calls will not be issued?
We want upon last real mode usage to leave all Xen PV information in a
clean state, this means, closing the channel and ring between the
newly created domain and the host system.
How can you be certain an OS won't switch back to real mode even
after an extended period of up-time? Or that such switching back
would affect you (could be calling e.g. the video or PCI BIOS
functions only).

There is INT15 AX=EC00 with BX specifying the target operating
mode, but that's apparently being called only before entering
long mode (i.e. wouldn't cover 32-bit OSes). And it would neither
be a guarantee that the OS might not later return to real mode.

Wouldn't it be possible for the BIOS to reestablish the connection to Xen
in this case? This might be the best solution: close the channel and ring
at some specific event (might even be timer based) and open them again
if really needed.


Juergen Gross                 Principal Developer Operating Systems
TSP ES&S SWE OS6                       Telephone: +49 (0) 89 3222 2967
Fujitsu Technology Solutions              e-mail: juergen.gross@xxxxxxxxxxxxxx
Domagkstr. 28                           Internet: ts.fujitsu.com
D-80807 Muenchen                 Company details: ts.fujitsu.com/imprint.html

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