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mysteries_intel: Add a section about SMM_BWP

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Something to point users to when SMM_BWP might be causing problems.

Change-Id: I394c033e8d4ff96433162f86aefb428d8acf6349
Signed-off-by: David Hendricks <david.hendricks@gmail.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/36986
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
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David Hendricks 2019-11-19 19:29:26 -08:00 committed by Nico Huber
parent 34d07f00b2
commit a9d6d1a817
1 changed files with 21 additions and 0 deletions
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Documentation/mysteries_intel.txt
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@ -5,6 +5,27 @@
A0h), so we have no clue if or where it is on ICH8. Out current policy is to
not touch it at all and assume/hope it is 0.
= SMM BIOS Write Protection =
Sometimes a hardware vendor will enable "SMM BIOS Write Protect" (SMM_BWP)
in the firmware during boot time. The bits that control SMM_BWP are in the
BIOS_CNTL register in the LPC interface.
When enabled, the SPI flash can only be written when the system is operating in
in System Management Mode (SMM). In other words, only certain code that was
installed by the BIOS can write to the flash chip. Programs that run in OS
context such as flashrom can still read the flash chip, but cannot write to the
flash chip.
Flashrom will attempt to detect this and print a warning such as the following:
"Warning: BIOS region SMM protection is enabled!"
Many vendor-supplied firmware update utilities do not actually write to the ROM;
instead they transfer data to/from memory which is read/written by a routine
running in SMM and is responsible for writing to the firmware ROM. This causes
severe system performance degradataion since all processors must be in SMM
context (ring -2) instead of OS context (ring 0) while the firmware ROM is being
written.
= Accesses beyond region bounds in descriptor mode =
Intel's flash image tool will always expand the last region so that it covers
the whole flash chip, but some boards ship with a different configuration.