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

Jan Beulich JBeulich at novell.com
Thu May 19 10:08:37 CEST 2011

>>> On 19.05.11 at 07:33, Daniel Castro <evil.dani at gmail.com> 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.


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