[M] Change in coreboot[main]: Documentation: Describe how SMMSTORE can be used safely

[Nico Huber (Code Review)]

Attention is currently required from: Benjamin Doron, Patrick Georgi.

Nico Huber has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/79095?usp=email )

Change subject: Documentation: Describe how SMMSTORE can be used safely
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Patch Set 2: Code-Review+1

(2 comments)

Patchset:

PS2: 
Thanks for the write-up!

I think it leaves a few questions open. But given the scope "Design
rationale", I see no need to bikeshed anything. If this is going to
be implemented, it may not be 100% what was proposed anyway. But this
is definitely a very good starting point.

File Documentation/drivers/smmstore.md:

https://review.coreboot.org/c/coreboot/+/79095/comment/40cd836a_f28b4b3c :
PS2, Line 145:   - while doing so, process authentication data and reject invalid blocks;
I always imagined something with two flash regions: One that is updated
during boot after authentication and write-protected from then on, and
one that is always writeable and filled by SMMSTORE. I believe such a
partial write protection is possible with both AMD and Intel chipsets.

I guess it would allow something like an authenticated CLEAR. We'd always
have the old version from the last boot in NVM.

If that sounds reasonable, maybe it's worth to mention here.