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

Anthony Liguori
2007-09-30T04:29:32Z

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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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