linux-kernel - Re: [PATCH v2 1/6] arm64: errata: Assume that unknown CPUs _are

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Message-ID: <CAODwPW_c+Ycu_zhiDOKN-fH2FEWf2pxr+FcugpqEjLX-nVjQrg@mail.gmail.com>
Date: Fri, 13 Dec 2024 18:25:34 -0800
From: Julius Werner <jwerner@...omium.org>
To: Douglas Anderson <dianders@...omium.org>
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, linux-arm-msm@...r.kernel.org, 
	Jeffrey Hugo <quic_jhugo@...cinc.com>, Julius Werner <jwerner@...omium.org>, 
	linux-arm-kernel@...ts.infradead.org, Roxana Bradescu <roxabee@...gle.com>, 
	Trilok Soni <quic_tsoni@...cinc.com>, bjorn.andersson@....qualcomm.com, 
	stable@...r.kernel.org, James Morse <james.morse@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/6] arm64: errata: Assume that unknown CPUs _are_
 vulnerable to Spectre BHB

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.