[coreboot] [PATCH] xmmstack fixes

Kevin O'Connor kevin at koconnor.net
Fri Oct 9 21:34:25 CEST 2009

On Fri, Oct 09, 2009 at 07:47:12PM +0200, Rudolf Marek wrote:
> I added support for pushl xxx(%esp) instruction and also I managed to create
> smarter wrapper code which means 10% drop in size (measured if -O0 is used).


BTW, I saw some further ways to enhance xmmstack:

It looks like either the xmm or mxx space is used.  However, my read
of the docs says both xmm and mxx can be used at the same time - this
could increase the space from 128 bytes to 192 bytes.

The code only fixes pointers relative to %esp, but I don't think
anything stops gcc from using non-esp pointers when referring to the
stack.  I think the code could be extended to fixup all memory
accesses.  Unfortunately, this would break those places where the code
needs to access real-memory (eg, pci space or constants in flash).  To
get around this, the wrapper functions could check the absolute
address of the access and only use xmm/mmx for those accesses between
say 0x0040-0x00c0 (where the stack could be located).  In order to test
the address, instructions that set flags would be needed, but backing
up and restoring the flags doesn't sound too hard (with lahf/"seto

Finally, it appears much of xmmstack is static code inserted at the
top of the file.  I think moving the static code into it's own
assembler file that is included might make what's going on a little
more clear.

Just some random thoughts..

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