Alternative option for solving Fedora i686 vs geode problems

Peter Robinson pbrobinson at gmail.com
Mon Jun 7 22:33:55 EDT 2010


On Tue, Jun 8, 2010 at 1:49 AM, John Gilmore <gnu at toad.com> wrote:
> I looked at the kernel patch for emulating the missing instruction
> (long NOP).  It looks like it works, and only needs minor patching-up
> for security enforcement.  The big argument on the Linux kernel list
> was about not having a little kludge like this, which is likely to
> grow to emulate many other things and become unmaintainable; some
> people would rather use the whole virtual-CPU emulation mechanism for
> this, which is much more heavyweight, but also a lot easier to test
> and validate.
>
> If having to maintain a 20- or 50-line kernel patch is the price of
> being able to move forward onto using unmodified modern Fedora release
> repositories, I'd say the choice for OLPC is very clear - do it.
>
> The change that introduced the use of this instruction was in the
> binutils (assembler) rather than in gcc.  I believe it is used when a
> .align directive is given in an executable segment.  When optimizing,
> GCC has been aligning some loops on cache line boundaries for some
> time (by inserting nop padding BEFORE the loop), but previously, the
> assembler was filling with various N-byte NOPs that would work on any
> x86.  It has been improved to pick faster ones on modern hardware.
> The relevant code is in gas/config/tc-i386.c, function
> i386_align_code().  I haven't pinned down the actual code change that
> bit us, and perhaps it's just the change in -march and/or -mtune
> options used by Fedora when calling gcc.  Gas is careful to only use
> NOPs that are compatible with the specified instruction set, if one is
> specified -- but Fedora is specifying the wrong one for our purposes.

The mail -march and/or -mtune options to compile the whole of Fedora
wasn't changed from F-12 to F-13 release so it was something else that
changed that triggered the issues. It was mentioned somewhere it was a
change in glibc. Whether it was glibc or something or if it was by
luck or something else that changed which meant it was seen only in
F-13 and not F-12 I don't have the deep technical understanding to
make that judgement.

Peter


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