libpayload: x86/exec - simplify and robustify the code

Simplify the code by directly using the arguments on the stack as base
pointer relative memory references, instead of loading them into
intermediate registers first.

Make it more robust by preserving all callee saved registers mandated by
the C calling convention (and only those), namely EBP, EBX, ESI and EDI.

Don't assume anything about the register state when the called function
returns -- beside the segment registers and the stack pointer to be
still the same as before the call.

Change-Id: I383d6ccefc5b3d5cca37a1c9b638c231bbc48aa8
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Reviewed-on: https://review.coreboot.org/18335
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)
1 file changed