Re: Ideas for PV on SeaBIOS

[Stefan Hajnoczi <stefanha@gmail.com>]

On Sat, May 21, 2011 at 2:29 PM, Kevin O'Connor <kevin@koconnor.net> wrote:
> 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)?

There ought to be a way for the guest to reset the device.  At least
in virtio this can be done and allows an in-use device to be reused,
no matter what state it is in.

Stefan