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Date: Thu, 25 Mar 2010 09:22:14 +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

On 03/24/10 23:28, Juan Quintela wrote:
> Humm, I think it means.  Can you migrate from a "new" vga to an old one,
> and maintain it working?

Depends on the vgabios version.

vgabios 0.6c will not support vesa gfx modes on older qemu, no matter 
whenever you started fresh or migrated to it.

> -    s->vbe_regs[VBE_DISPI_INDEX_ID] = VBE_DISPI_ID0;
> +    s->vbe_regs[VBE_DISPI_INDEX_ID] = VBE_DISPI_ID5;
> +    s->vbe_regs[VBE_DISPI_INDEX_VIDEO_MEMORY_64K] = s->vram_size / (64 * 1024);

> After migration, vbe_regs[VBE_DISPI_INDEX_ID] would have the value
> VBE_DISPI_ID5, but vbe_regs[VBE_DISPI_INDEX_VIDEO_MEMORY_64K] will have
> any random value, no?

vgabios uses both once at init time.  resetting vga will reset the vbe 
regs too.  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.

cheers,
   Gerd