[Qemu-devel] Re: [PATCH] update bochs vbe interface

[Juan Quintela <quintela@redhat.com>]

Gerd Hoffmann <kraxel@redhat.com> wrote:
> 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. 

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

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

> resetting vga will reset the vbe
> regs too.

Yes, but I guess you agree that forcing a reset in the middle of a live
migration makes the whole point of migration moot :-)

>  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 ....

> cheers,
>   Gerd