sec/intel/txt: Split MTRR setup ASM code into a macro

If necessary, SCLEAN needs to run in early romstage, where DRAM is not
working yet. In fact, that the DRAM isn't working is the reason to run
SCLEAN in the first place. Before running GETSEC, CAR needs to be torn
down, as MTRRs have to be reprogrammed to cache the BIOS ACM. Further,
running SCLEAN leaves the system in an undefined state, where the only
sane thing to do is reset the platform. Thus, invoking SCLEAN requires
specific assembly prologue and epilogue sections before and after MTRR
setup, and neither DRAM nor CAR may be relied upon for the MTRR setup.

In order to handle this without duplicating the MTRR setup code, place
it in a macro on a separate file. This needs to be a macro because the
call and return instructions rely on the stack being usable, and it is
not the case for SCLEAN. The MTRR code clobbers many registers, but no
other choice remains when the registers cannot be saved anywhere else.

Tested on Asrock B85M Pro4, BIOS ACM can still be launched.

Change-Id: I2f5e82f57b458ca1637790ddc1ddc14bba68ac49
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46603
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

0 files changed, 0 insertions, 0 deletions