Re: FreeBSD boot loader on VT-x based Xen system

[Andy Clayton <xen.nospam@q3aiml.net>]

Andy Clayton wrote:
> Randy Thelen wrote:
>> Folks --
>>
>> I'm trying to run FreeBSD 6.0 on a VT-x based Xen system.  I'm 
>> experiencing a problem where the boot loader is crashing before the 
>> FreeBSD kernel (proper) is loaded.
>>
>
> I don't think I can help you much as far as fixing, but earlier I 
> opened a bug report in Bugzilla regarding this issue at 
> http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=622
> I will try to help any way I can but I have next to no experience with 
> assembly.  Time to learn, perhaps.
>
> Good luck for now and ask if you would like some help (with something 
> I can actually help with...),
> Andy Clayton
>
Hey there is a patch that was mailed to -devel by Kevin Tronkowski today 
called "[Xen-devel] [PATCH] Extensions to vmxassist to handle additional 
mode switch cases".  It sounds promising. 

If I may quote:

Attached is a patch which extends vmxassist to handle two cases 
related to mode switching found while experimenting with different 
boot loaders.

The first case is use of the JMP instruction with memory location 
operands to complete a switch to protected mode. This patch adds 
emulation for this form of the JMP instruction to vmxassist.

The second case is where boot loader code does not save/restore a 
non-zero SS register across a protected mode traversal. 
Zeroing the SS register in vmxassist results in all sorts of problems 
in the domU after returning back to real mode. This patch stores 
segment register values before entering protected mode and correctly 
restores the old values (instead of an incorrect zero value) when 
reentering real mode. 

The first case is what I was looking at.  I'll see if I can check this 
out shortly...

Andy Clayton