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

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

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Hi Steffen,

As Avi pointed out, VT requires that SS.RPL == CS.RPL.  We're seeing 
gfxboot fail under KVM because ss = 0x5761 while cs = 0x4004 during the 
transition from real mode to protected mode.  The attached patch passes 
the value of ss through ebx since KVM has to sanitize the value of ss to 
make VT happy.

I've tested this with a remastered Ubuntu Gutsy install CD.  I couldn't 
find the right gfxboot theme for the openSuSE install CD I have so I 
wasn't able to test it.

I suspect that Xen should have a very similar problem as I can't think 
of a possible way to work around this.

Regards,

Anthony Liguori

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Subject: [PATCH] Fix gfxboot under VT
From: Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>

This patch lets gfxboot-3.3.38 work under KVM.  The fix was suggested by Avi
Kivity.

Signed-off-by: Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>

diff -ur a/bincode.asm b/bincode.asm
--- a/bincode.asm	2007-07-24 05:49:46.000000000 -0500
+++ b/bincode.asm	2007-09-29 22:14:35.000000000 -0500
@@ -15519,6 +15519,7 @@
 switch_to_pm:
 		pushf
 		push eax
+		push ebx
 
 		mov eax,cr0
 
@@ -15534,6 +15535,11 @@
 		mov word [cs:rm_seg.fs],fs
 		mov word [cs:rm_seg.gs],gs
 
+		;; ss:rpl must equal cs:rpl in PM for VT.  we can't rely on ss
+		;; maintaining it's value after the transition so we have to
+		;; pass it in a GP register
+		mov ebx,ss
+	
 		or al,1
 		o32 lgdt [cs:pm_gdt]
 		o32 lidt [cs:pm_idt]
@@ -15546,7 +15552,7 @@
 		mov ax,pm_seg.prog_d16
 		mov ds,ax
 
-		mov eax,ss
+		mov eax,ebx
 		and esp,0ffffh
 		shl eax,4
 		add esp,eax
@@ -15557,6 +15563,7 @@
 		mov fs,ax
 		mov gs,ax
 
+		pop ebx
 		pop eax
 		popfw
 		o16 ret

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