[SeaBIOS] [BUG] Windows 7 fails to start with new vgabios binaries

Kevin O'Connor kevin at koconnor.net
Mon Dec 9 23:12:51 CET 2013

On Mon, Dec 09, 2013 at 10:57:25AM -0500, Kevin O'Connor wrote:
> On Mon, Dec 09, 2013 at 02:20:59PM +0100, Gerd Hoffmann wrote:
> > On Mo, 2013-12-09 at 14:18 +0200, Gal Hammer wrote:
> > > A Windows 7 (32-bit) VM running with QXL device fails to start with the 
> > > new updated vgabios binaries (commit 
> > Tracked down to the new vgabios stack switching.  With
> > CONFIG_VGA_ALLOCATE_EXTRA_STACK=n everything is fine again.
> Ughh.  WinXP doesn't have the problem, but I can also reproduce on
> Win7 beta.
> I'll change the default for CONFIG_VGA_ALLOCATE_EXTRA_STACK to off.

I tracked this down further.  The problem is the Windows x86 emulator
doesn't correctly handle memory accesses relative to the %esp
register.  Julian reported this some time back and we worked around it
then.  However, the recent "extra stack" assembler code inserted a few
of these instructions.

I think the proper fix is to leave CONFIG_VGA_ALLOCATE_EXTRA_STACK on
and use slightly different assembler so as not to aggravate win7.

I tested the seabios patch below and my win7 beta now boots okay.


--- a/vgasrc/vgaentry.S
+++ b/vgasrc/vgaentry.S
@@ -97,12 +97,9 @@ entry_10_extrastack:
         movl %ecx, BREGS_ecx(%eax)
         movw %es, BREGS_es(%eax)
         movl %esp, BREGS_size+0(%eax)
-        movzwl %sp, %esp
         movw %ss, BREGS_size+4(%eax)
-        movl (%esp), %edx
-        movl %edx, BREGS_code(%eax)
-        movw 4(%esp), %dx
-        movw %dx, BREGS_flags(%eax)
+        popl BREGS_code(%eax)
+        popw BREGS_flags(%eax)
         movw %ds, %dx           // Setup %ss/%esp and call function
         movw %dx, %ss

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