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

[Kevin O'Connor]

On Thu, May 19, 2011 at 09:08:37AM +0100, Jan Beulich wrote:
> >>> 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).

I don't think any modern OS will call the int13 (disk drive) functions
once the OS has booted.  Doing so would be highly error prone even on
real hardware.  Once the OS has registered it's own driver for a disk,
it can't go back to the BIOS handlers for it.

-Kevin