Re: [SeaBIOS] FreeBSD VESA console not working with SeaBIOS

[Jon Doe]

On Thu, Apr 9, 2015 at 6:54 PM, Kevin O'Connor  wrote:
> On Thu, Apr 09, 2015 at 09:56:07AM -0400, Kevin O'Connor wrote:
> > On Wed, Apr 08, 2015 at 10:32:23PM -0400, Kevin O'Connor wrote:
> > > The problem is not with leal in hand written assembler - the problem
> > > is with leal instructions generated by gcc.  To see the assembler gcc
> > > produces for the vgabios one can look at out/vgaccode16.raw.s .  Or,
> > > alternatively, one can run:
> > >   objdump -m i386 -M i8086 -M suffix -ldr out/vgarom.o
> > >
> > > We've fixed up gcc assembler in the past (see scripts/vgafixup.py) to
> > > work around x86emu.  However, the leal instruction seems painful to
> > > patch out - particularly so when %esp is one of the registers read or
> > > written in the leal instruction.  If anyone wants to take a stab
> > > at a workaround, feel free to submit a patch.
> >
> > It occurred to me that it might be possible to replace the leal
> > instruction with a function call.  The generic form of leal is:
> >
> > leal $offset(%base, %index, $scale), %destination
> >
> > which does:
> >
> > %destination = $offset + %base + (%index * $scale)
> >
> > if the above is found in the gcc assembler it could be replaced with
> > something like:
> >
> > pushl %base
> >   pushl %index
> >   pushl $offset
> >   pushl $scale
> >   callw fake_leal
> >   popl %destination
> >
> > (The fake_leal assembler function that performs the actual calculation
> > would also need to be written.)  The above wouldn't work if %index is
> > %esp, but I don't think that would happen in practice.
> >
> > I'm not going to tackle the above right now, but maybe someone else is
> > interested in attempting it.  A word of caution though - I know leal
> > doesn't work in very old x86emu versions, but it's unclear if any
> > other gcc produced instructions are also broken.  The leal instruction
> > might just be one of many instructions that don't work.
>
> I gave it a shot anyway - seabios patch is below.
>
> It's incredibly ugly.  But, it does allow FC11 and FC12 to boot into
> graphics which wasn't possible before.  Also, I don't see any
> indication in the x86emu commit logs (
> http://anongit.freedesktop.org/git/xorg/xserver.git ) that any other
> instructions besides leal will present a problem.
>
> -Kevin
>
> commit c900162603f19552ec8ad3648bbb71f2effc227d
> Author: Kevin O'Connor 
> Date:   Thu Apr 9 11:59:41 2015 -0400
> vgabios: Hack for leal emulation
>
> Signed-off-by: Kevin O'Connor <[e-mail]>
>
> diff --git a/scripts/vgafixup.py b/scripts/vgafixup.py
> index a981bbf..f32c055 100644
> --- a/scripts/vgafixup.py
> +++ b/scripts/vgafixup.py
> @@ -8,19 +8,26 @@
>  # The x86emu code widely used in Linux distributions when running Xorg
>  # in vesamode is known to have issues with "retl", "leavel", "entryl",
>  # and some variants of "calll".  This code modifies those instructions
> -# (ret and leave) that are known to be generated by gcc to avoid
> -# triggering the x86emu bugs.
> +# that are known to be generated by gcc to avoid triggering the x86emu
> +# bugs.  It also translates "leal" instructions into a function call
> +# that emulates it at runtime.
>
> # It is also known that the Windows vgabios emulator has issues with
>  # addressing negative offsets to the %esp register.  That has been
>  # worked around by not using the gcc parameter "-fomit-frame-pointer"
>  # when compiling.
>
> -import sys
> +import sys, re
>
> def main():
>      infilename, outfilename = sys.argv[1:]
>      infile = open(infilename, 'r')
> re_leal = re.compile(
>    r'^(?P<offset>[^(]*)\('
>    r'(?P<base>[^,)]*)'
>    r'(?:,(?P<index>[^,)]*))?'
>    r'(?:,(?P<scale>[^,)]*))?\)'
>    r', (?P<dest>.*)$')
> out = []
>    for line in infile:
>        sline = line.strip()
>
> @@ -30,6 +37,25 @@ def main():
>              out.append('movl %ebp, %esp ; popl %ebp\n')
>          elif sline.startswith('call'):
>              out.append('pushw %ax ; callw' + sline[4:] + '\n')
>    elif sline.startswith('leal'):
>        m = re_leal.match(sline[5:])
>        if m is None or m.group('index') == '%esp':
>            print("Invalid leal instruction: %s" % (sline,))
>            sys.exit(-1)
>        offset, base, index, scale, dest = m.group(
>            'offset', 'base', 'index', 'scale', 'dest')
>        if not offset:
>            offset = '0'
>        if not base:
>            base = '$0'
>        if not index:
>            index = '$0'
>        if not scale:
>            scale = '1'
>        scale = {'1': 0, '2': 1, '4': 2, '8': 3}[scale]
>        out.append('pushl %s ; pushl %s ; pushl $%s ; pushw $%d'
>                   ' ; callw fake_leal ; popl %s\n' % (
>                       base, index, offset, scale, dest))
>    else:
>        out.append(line)
> infile.close()
>
> diff --git a/vgasrc/vgaentry.S b/vgasrc/vgaentry.S
> index e0ab954..21128c5 100644
> --- a/vgasrc/vgaentry.S
> +++ b/vgasrc/vgaentry.S
> @@ -45,31 +45,32 @@ _rom_header_signature:
> Entry points
>
> ****************************************************************/
>    // Force a fault if found to be running on broken x86emu versions.
>    DECLFUNC x86emu_fault
>
> -msg:    .ascii "SeaVGABIOS: x86emu leal trap!\n"
> -x86emu_fault:
> -#if CONFIG_DEBUG_IO
>    movw %cs:DebugOutputPort, %dx
>    movw $msg, %si
>
> -1:      movb %cs:(%si), %al
>    outb %al, (%dx)
>    incw %si
>    cmpw $x86emu_fault, %si
>    jl 1b
>
> -#endif
> -1:      hlt
>    jmp 1b
>    // Emulate a leal instruction (see scripts/vgafixup.py)
>    // On entry stack contains: base, index, offset, scale
>    DECLFUNC fake_leal
>    .global fake_leal
>
> +fake_leal:
>    pushl %ebp
>    movl %esp, %ebp
>    pushfl
>    pushl %eax
>    pushl %ecx
>    movb 6(%ebp), %cl
>    movl 12(%ebp), %eax
>    shll %cl, %eax
>    addl 8(%ebp), %eax
>    addl %eax, 16(%ebp)
>    popl %ecx
>    popl %eax
>    popfl
>    popl %ebp
>    retw $10
>
>    // 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

I can report that this patch also works for FreeBSD 8.2's VESA
console. There seems to be some unrelated problem when running
1.8-stable so I applied it to rel-1.7.5.2