[SeaBIOS] PC-BSD installer does not boot with 1.7.4 (bisected)

Kevin O'Connor 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 -
> see:
> http://lists.gnu.org/archive/html/qemu-stable/2013-03/msg00037.html
> (*) 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.

Comments welcome.

commit e3caa553e940efb6184b30a5637134c4aa9f8b65
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
--- a/vgasrc/vgaentry.S
+++ b/vgasrc/vgaentry.S
@@ -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
+1:      hlt
+        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
+        .endm
+        // 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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