Re: [GFXBOOT] [PATCH] When switching to real-mode, pass SS in a GP register

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

Steffen Winterfeldt wrote:
> Hi,
>
> sorry for the delay, but I've been on vacation. :-)
>   

No worries :-)

>> You're right.  Good catch!
>>     
>
> Actually that is not true. 'mov eax,ss' does implicitly clear the upper
> 16 bits (both processor docs and hardware agree here).
>   

I wasn't able to find anything definitive in my manuals but I didn't 
look very hard.  I figured that erring on the safe side is better anyway.

> Anyway, ss is already saved, so no need for an extra register. Here is
> my version (tested and works on my machine):
>   

This patch works for me under KVM.  Thanks!

Regards,

Anthony Liguori

> --- bincode.asm	(revision 650)
> +++ bincode.asm	(working copy)
> @@ -15546,7 +15546,11 @@
>  		mov ax,pm_seg.prog_d16
>  		mov ds,ax
>  
> -		mov eax,ss
> +		; needed for KVM:
> +		; ss:rpl must equal cs:rpl in PM for VT. We can't rely on ss
> +		; maintaining its value after the transition.
> +
> +		movzx eax,word [rm_seg.ss]
>  		and esp,0ffffh
>  		shl eax,4
>  		add esp,eax
>
>   


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