Re: beginnings of allowing more than the basic 80x25 VGAscreen resolution

[David Hopwood <david.nospam.hopwood@blueyonder.co.uk>]

Keir Fraser wrote:
> On 5 Jul 2005, at 13:58, Gerd Knorr wrote:
> > "Ling, Xiaofeng" <xiaofeng.ling@intel.com> writes:
> 
>>> Is it possible to add some real mode code to xen like video.S in linux
>>> kernel to switch mode?
>>
>> Hmm, how about adding that to the boot loader(s)?
>>
>> IIRC there are some fields in the multiboot info header for that kind
>> of stuff, so grub could setup that and pass on the info to xen (and
>> xen in turn to the dom0 kernel?).

Yes. How this is supposed to work is that the OS's multiboot header
specifies a desired mode:
<http://www.gnu.org/software/grub/manual/multiboot/multiboot.html#Header%20graphics%20fields>
and then the boot loader passes back information about the actual mode
that was set in the vbe_* fields of the information struct.

>> multiboot kernels are started in 32-bit protected mode by the boot
>> loader, i.e. xen has (unlike linux) no 16-bit startup code.  Thus
>> doing 16-bit BIOS calls once xen is running is a bit difficult ...
> 
> GRUB has code for switching back into real mode to do bios calls. We 
> could take that, plus video.S, and put them in Xen. Then add an info 
> structure for passing video info up to domain0. It's pretty ugly, but I 
> think there is no nice solution here. Would we also need vesa/vga driver 
> code from Linux for setting up font data in some graphics modes?

I prefer Gerd's suggestion of relying on the boot loader to set up a
reasonable mode. Multiboot is all about providing a sane initial hardware
state to multiboot-compliant kernels, so that they do not need as much
(preferably not any) of this crufty 16-bit initialization code.
This is code that with a cleaner hardware/BIOS design would not be needed
at all; ideally, OS designers should be able to forget about it like a
bad dream.

On current PC hardware, a boot loader must have code for doing BIOS calls
anyway, so it might as well set up the video mode (and font if needed).
I don't know what modes GRUB currently supports, but if it doesn't support
the > 25-line text modes then that's an RFE for GRUB, not something that
needs to be fixed by adding more cruft to Xen.

-- 
David Hopwood <david.nospam.hopwood@blueyonder.co.uk>