[PATCH v2 1/6] arm64: errata: Assume that unknown CPUs _are_ vulnerable to Spectre BHB

[Julius Werner]

I feel like this patch is maybe taking a bit of a wrong angle at
achieving what you're trying to do. The code seems to be structured in
a way that it has separate functions to test for each of the possible
mitigation options one at a time, and pushing the default case into
spectre_bhb_loop_affected() feels like a bit of a layering violation.
I think it would work the way you wrote it, but it depends heavily on
the order functions are called in is_spectre_bhb_affected(), which
seems counter-intuitive with the way the functions seem to be designed
as independent checks.

What do you think about an approach like this instead:

- Refactor max_bhb_k in spectre_bhb_loop_affected() to be a global
instead, which starts out as zero, is updated by
spectre_bhb_loop_affected(), and is directly read by
spectre_bhb_patch_loop_iter() (could probably remove the `scope`
argument from spectre_bhb_loop_affected() then).

- Add a function is_spectre_bhb_safe() that just checks if the MIDR is
in the new list you're adding

- Add an `if (is_spectre_bhb_safe()) return false;` to the top of
is_spectre_bhb_affected

- Change the `return false` into `return true` at the end of
is_spectre_bhb_affected (in fact, you can probably take out some of
the other calls that result in returning true as well then)

- In spectre_bhb_enable_mitigations(), at the end of the long if-else
chain, put a last else block that prints your WARN_ONCE(), sets the
max_bhb_k global to 32, and then does the same stuff that the `if
(spectre_bhb_loop_affected())` block would have installed (maybe
factoring that out into a helper function called from both cases).

I think that should implement the same "assume unsafe by default
unless explicitly listed as safe, check for all other mitigations
first, then default to k=32 loop if none found" approach, but makes it
a bit more obvious in the code.