[SeaBIOS] [PATCH 1/3] vgabios: Add config option for assembler fixups

[Kevin O'Connor]

On Sat, Apr 11, 2015 at 08:58:20AM +0200, Paul Menzel wrote:
> Am Freitag, den 10.04.2015, 11:07 -0400 schrieb Kevin O'Connor:
> > Add a kconfig build option (CONFIG_VGA_FIXUP_ASM) to allow users to
> > build the vgabios without the complex assembler fixups that work
> > around emulator bugs.
> > 
> > Signed-off-by: Kevin O'Connor <kevin at koconnor.net>
> > ---
> >  Makefile          | 21 +++++++++++----------
> >  vgasrc/Kconfig    | 10 ++++++++++
> >  vgasrc/vgaentry.S |  4 ++++
> >  3 files changed, 25 insertions(+), 10 deletions(-)
> > 
> > diff --git a/Makefile b/Makefile
> > index a84a5f7..a4d945c 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -56,7 +56,7 @@ COMMONCFLAGS := -I$(OUT) -Isrc -Os -MD -g \
> >      -Wall -Wno-strict-aliasing -Wold-style-definition \
> >      $(call cc-option,$(CC),-Wtype-limits,) \
> >      -m32 -march=i386 -mregparm=3 -mpreferred-stack-boundary=2 \
> > -    -minline-all-stringops \
> > +    -minline-all-stringops -fomit-frame-pointer \
> 
> could you elaborate why `-fomit-frame-pointer` causes issues and needs
> to be moved around?

The x86 vgabios interpretter on some versions of Windows gets confused
when addresses relative to %esp are used (eg, "movl 4(%esp), %eax").
See 9887ecb7 and 41c6061d.  In practice, -fno-omit-frame-pointer
causes gcc to not emit these addresses.

> > +    config VGA_FIXUP_ASM
> > +        bool "Fixup assembler to work with broken emulators"
> > +        default y
> > +        help
> > +            This option will cause the build to attempt to avoid
> > +            certain x86 machine instructions that are known to confuse
> > +            some emulators.  In particular, it works around
> > +            deficiencies in the Windows vgabios emulator and the
> 
> Use Microsoft Windows?

Was that unclear?  I prefer not to call out company names in defect
descriptions.

-Kevin