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

Keir Fraser keir.xen at gmail.com
Thu May 19 09:19:17 CEST 2011

On 19/05/2011 06:33, "Daniel Castro" <evil.dani at gmail.com> wrote:

> Good Day,
> 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.
> If you have any ideas please let me know.

There's no easy way. Best effort might be to hook off the guest OS setting
up its PV drivers. One of the first steps of that would be getting a
hypercall transfer page, and also setting up event-channel delivery. It may
be necessary for the hypervisor to give the BIOS some help by delivering a
pre-registered callback on one of those events, to clean up. This is made
uglier by the fact you don't know what execution mode the OS might be in
when it triggers the callback. Needs a bit more thought.

 -- Keir

> Daniel

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