[x86][seses] Add documentation for SESES

[zbrid]

Summary

This patch includes documentation about the pass Speculative Execution
Side Effect Suppression. The document describes Load Value Injection,
the mitigation, and all the experiments ran on various LFENCE
placements.

One pager on Load Value Injection:
https://software.intel.com/security-software-guidance/software-guidance/load-value-injection

Deep dive on Load Value Injection:
https://software.intel.com/security-software-guidance/insights/deep-dive-load-value-injection

Impl patch: https://reviews.llvm.org/D75939

[craig.topper]

llvm/docs/SpeculativeExecutionSideEffectSuppression.md

545

These commit ids are only valid for you local copy right? And would change any time you rebase.

[craig.topper]

llvm/docs/SpeculativeExecutionSideEffectSuppression.md

545

Or is this the baseline revision you applied your patches against?

[zbrid]

Clarify what the Git commit IDs in the benchmark section refers to

[zbrid]

Updated based on comments. Thanks for the feedback!

llvm/docs/SpeculativeExecutionSideEffectSuppression.md

545

This the baseline revision I applied my patches against when I ran the benchmarks. I'll update the documentation to clarify that.

[zbrid]

Update with info about this being a big hammer not only for LVI

[zbrid]

@craig.topper Any other comments?

[zbrid]

[seses][docs] Mention req to use -mlvi-cfi; Format code-block

[zbrid]

@craig.topper @george.burgess.iv Friendly ping on this documentation.

[mattdr]

Blast from the past. It seems like there's still benefit in checking in this documentation, appropriately updated to reflect the fact that SESES is now actually in the tree.

But I think we should take a second look at how much detail to provide about variants of the mitigation. As it stands, we don't have solid advice for how to make the tradeoff between them, and if someone asked we'd tell them to use the defaults. Yet 50%+ of this doc is spent covering those variations.

Maybe it's enough to gesture vaguely in the direction of experimental options, but leave out the figures and discussion?

llvm/docs/SpeculativeExecutionSideEffectSuppression.md

18–19

need to update this to refer to the other LVI mitigation that's been checked in.

75

update wording now that SESES is committed