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Re: [Xen-devel] Windows 8 VGA Passthrough

On Sunday, October 27, 2013 1:21 PM, John Wargo wrote:
>I've made quite a bit of changes since my last email, and so far here's what 
>I've got:
>AMD FX6300
>ASUS M5A99X evo r2.0
>ASUS Radeon 6670 (Dom0)
>XFX Radeon 7850 (Win8)
>Is there some sort of AMD Radeon patch I need to use (I saw one on the wiki, 
>but it appeared to be several years old), or is it a needed configuration?
>Thanks for your time,

Hi John,

I am using XEN4.2 with QEMU-traditional and I have identified a couple of 
issues in regards to passing through a video adapter as the primary (VGA 
enabled) adapter.

The patch that you referenced attempts to get around one of the issues but it 
is not complete and doesn't cover all of the cases.  If you apply the patch it 
will definitely help but not guaranteed to fix all of your problems.

The second issue is related to the fact that you are using two different 
graphics adapters.  I am assuming that the Radeon 6670 is the primary 
(boot/VGA) adapter and that the 7850 is the secondary.

From reading your thread I assume that you want to pass the 7850 through to the 
DomU as the primary.  I have been able to do this successfully with a few 
fixes.   The problem is in the way QEMU/hvmloader handle the VBios code.  When 
you boot with the 6670 as the primary the VBios code for the graphics adapter 
is copied to memory at address 0xc0000.  When QEMU passes the 7850 through to 
the DomU it copies VBios from 0xc0000 in Dom0 memory space into 0xc0000 in DomU 
memory space.  DomU is effectively executing the VBios for the wrong video 
card.  The correct fix is to have the hvmloader read a fresh copy of VBios from 
the 7850 and copy it into the DomU address space at 0xc0000.

Sometimes you can get away with executing the wrong VBios, sometimes you get 

Hope the explanation helps,

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