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Re: [Qemu-devel] [SeaBIOS] [BUG] Windows 7 fails to start with new vgabi
Re: [Qemu-devel] [SeaBIOS] [BUG] Windows 7 fails to start with new vgabios binaries
Mon, 9 Dec 2013 17:12:51 -0500
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.
@@ -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