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

[Kevin O'Connor <kevin@xxxxxxxxxxxx>]

On Thu, May 19, 2011 at 02:33:52PM +0900, Daniel Castro 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?

In the general case, no.

The ACPI spec does define a mechanism for the OS to inform the BIOS
that it is transitioning from "Legacy state" to "Working state" via an
SMI.  SeaBIOS does have code for this (see src/smm.c), but it doesn't
currently do anything interesting.  Unfortunately, this is only
available for OSs that support ACPI.

> 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.

Is it required to close the channel?  Can opening a new channel
implicitly close the old channel (I believe this is what the virtio
stuff does)?

-Kevin

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