[SeaBIOS] PC-BSD installer does not boot with 1.7.4 (bisected)
kevin at koconnor.net
Thu Feb 6 05:04:42 CET 2014
On Wed, Feb 05, 2014 at 12:44:06PM -0500, Kevin O'Connor wrote:
> On Tue, Feb 04, 2014 at 04:33:19PM +0400, Michael Tokarev wrote:
> > We have a bugreport in debian, http://bugs.debian.org/737142,
> > stating that PC-BSD does not work with seabios-1.7.4 anymore.
> > I digged in, and found out that it fails only with -vga std
> > (cirrus works fine). So I bisected the issue - only changing
> > vgabios-stdvga.bin, and found this:
> This seems to be similar to the freebsd problem reported last year -
> (*) The best I can think of is to try and construct some hand crafted
> assembler that can catch buggy x86emu emulators and force a failure..
I have put together some assember to try and run-time test for known
broken versions of x86emu. This patch to SeaVGABIOS does seem to
catch the freebsd issue and in my tests it convinces freebsd to take a
different approach. The patch is a bit ugly though.
Author: Kevin O'Connor <kevin at koconnor.net>
Date: Wed Feb 5 22:47:29 2014 -0500
vgabios: Attempt to detect old x86emu and force a fault.
Check for cases where the leal instruction does not work. This
instruction is known to not be emulated properly on old versions of
x86emu. If a broken version of x86emu is found, force a fault that
x86emu will easily detect. This should help prevent soft failures
when running old software.
Signed-off-by: Kevin O'Connor <kevin at koconnor.net>
diff --git a/vgasrc/vgaentry.S b/vgasrc/vgaentry.S
index 9854448..e246e7c 100644
@@ -45,9 +45,27 @@ _rom_header_signature:
* Entry points
- // This macro is the same as ENTRY_ARG except the "calll"
- // instruction is avoided to work around known issues in the
- // emulation of some versions of x86emu.
+ // Force a fault if found to be running on broken x86emu versions.
+ DECLFUNC x86emu_fault
+ int $0x03
+ jmp 1b
+ // This macro implements a call while avoiding instructions
+ // that old versions of x86emu have problems with.
+ .macro VGA_CALLL cfunc
+ // Make sure leal instruction works.
+ movl $0x8000, %ecx
+ leal (%ecx, %ecx, 1), %ecx
+ cmpl $0x10000, %ecx
+ jne x86emu_fault
+ // Use callw instead of calll
+ push %ax
+ callw \cfunc
+ // This macro is the same as ENTRY_ARG except VGA_CALLL is used.
.macro ENTRY_ARG_VGA cfunc
@@ -57,7 +75,7 @@ _rom_header_signature:
movl %esp, %ebx // Backup %esp, then zero high bits
movzwl %sp, %esp
movl %esp, %eax // First arg is pointer to struct bregs
- pushw %ax ; callw \cfunc
+ VGA_CALLL \cfunc
movl %ebx, %esp // Restore %esp (including high bits)
@@ -103,7 +121,7 @@ entry_10_extrastack:
movw %ds, %dx // Setup %ss/%esp and call function
movw %dx, %ss
movl %eax, %esp
- pushw %ax ; callw handle_10
+ VGA_CALLL handle_10
movl %esp, %eax // Restore registers and return
movw BREGS_size+4(%eax), %ss
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