Re: [Qemu-devel] [PATCH v4 0/6] save/restore on Xen

[Anthony Liguori <anthony@codemonkey.ws>]

On 01/24/2012 05:52 AM, Stefano Stabellini wrote:
> On Tue, 24 Jan 2012, Avi Kivity wrote:
>> On 01/24/2012 01:27 PM, Paolo Bonzini wrote:
>>> On 01/24/2012 12:10 PM, Avi Kivity wrote:
>>>>> But viewing RAM as just another device, having Xen only restore a
>>>>> subset of devices should be a reasonable thing to do moving forward.
>>>>> The main problem here I believe is that we have part of the VGA Bios
>>>>> functionality in the hardware emulation.
>>>>
>>>> Doesn't the main BIOS clear the screen first thing at boot?  Not even
>>>> sure the reset is needed.
>>>
>>> Clearing the screen should only write to the RAM at 0xB8000 (and
>>> perhaps 0xA0000 since IIRC it's where text-mode fonts lie).  The
>>> option ROM cannot even assume that the main BIOS knows about the VESA
>>> framebuffer, can it?
>>
>> Yes, but why should anything else be needed?
>>
>> When you switch to a graphics mode, clear as much of the framebuffer as
>> you need.
>
> After installing Win2K, I managed to reproduce the issue with the old
> qemu-xen and rombios (removing the memset in the vga emulator).
> However I cannot reproduce the issue with upstream qemu and seabios
> (even if I remove the memset).
>
> I think that the memset was a workaround a bug in rombios, hence it is
> not needed anymore.
>
> Removing the cirrus_vga memset would solve the main issue we have left
> with save/restore on Xen.
> However it wouldn't solve the problem for QXL: as Gerd pointed out, QXL
> needs a reset to initialize some memory regions, so another "if we are
> restoring don't do that" would be required to make it work on Xen...

I believe this is a bug in QXL (or misdesign).  It should move this logic into 
the VGA Bios.

Regards,

Anthony Liguori