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Date: Thu, 25 Mar 2010 10:10:34 +0100
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: [Qemu-devel] Re: [PATCH] update bochs vbe interface
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To: Juan Quintela <quintela@redhat.com>
Cc: qemu-devel@nongnu.org

   Hi,

> Then our big problem is migration between read of the 1st register and
> of the 2nd register, no?

"big"?  The window is quite small, and I think we have a bunch of 
simliar issues elsewhere in qemu.  They are hardly avoidable for new -> 
old migration when adding new features to emulated devices.

Well, maybe sections can fix it, but probably only in case the old qemu 
is new enougth that it can handle sections too.

> Furthermore, older vga bios, seing VBE_DISPI_ID5, what are they going to
> do?

Work as they did before ;)

>>   So when migrating from new to old qemu:  Before reset
>> vgabios will have the video memory size saved somewhere.  After reset
>> ID will reset to ID0, and in case you are running vgabios 0.6c vesa
>> gfx modes will stop working.
>
> I see this part, but I still think that we have a window where we can be
> in very bad shape, no?  I guess that we don't support anything different
> that vgabios, so ....

See above.  Worst case is that vesa graphics modes stop working, even if 
you hit the race window (vgabios will think you have 0 MB video ram then 
and refuse all gfx modes).

cheers,
   Gerd