Re: [PATCH] x86/mm: compile out __set_fixmap_x() when not needed

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.11.2026 08:23, Nicola Vetrini wrote:
> On 2026-11-03 12:28, Jan Beulich wrote:
>> On 03.11.2026 10:42, Jan Beulich wrote:
>>> Only HYPERV_GUEST has a need for an executable fixmap, so with that 
>>> option
>>> unset __set_fixmap_x() (and l1_fixmap_x[] in turn) is unreachable. 
>>> Address
>>> this Misra rule 2.1 violation by introduction ofa new FIXMAP_X Kconfig
>>> setting.
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>> ---
>>> Why is __set_fixmap_x() not found by Eclair as a rule 2.1 violation in 
>>> the
>>> x86_64-amd job?
>>
>> Same question actually for both functions converted to static by [1]. 
>> This
>> apparently being a wider problem, are we (again / still) having an 
>> issue
>> with the scanning of the linking steps?
>>
>> Jan
>>
>> [1] https://lists.xen.org/archives/html/xen-devel/2026-10/msg01537.html
> 
> It was a Xen project decision not to consider for R2.1 functions that 
> are never referenced: you can see in 
> "automation/eclair_analysis/ECLAIR/deviations.ecl". Of course that is 
> reversible and I am in favour of not hiding those violations, but the 
> rationale for the justification that was given is that the compiler will 
> indeed DCE those:
> 
>     * - R2.1
>       - The compiler implementation guarantees that the unreachable code 
> is
>         removed. Constant expressions and unreachable branches of if and 
> switch
>         statements are expected.
>       - Tagged as `safe` for ECLAIR.

For the cases at hand (and then likely many more) the compiler doesn't DCE
anything. Imo the wording here is completely bogus. It may say "If the
compiler implementation guarantees ..." Which it might do for static
functions, but aiui without LTO it never will for non-static ones.

Then I'm also puzzled by the deviations.ecl entry:

-doc_begin="The compiler implementation guarantees that the unreachable code is 
removed.
Constant expressions and unreachable branches of if and switch statements are 
expected."
-config=MC3A2.R2.1,+reports={safe,"first_area(^.*has an invariantly.*$)"}
-config=MC3A2.R2.1,+reports={safe,"first_area(^.*incompatible with labeled 
statement$)"}
-doc_end

First - these are two different deviations, and hence imo should be two
separate entries. More important here though is that I can't connect the
two config= lines to the deviation of unreferenced non-static functions.
There's nothing "... has an invariantly ..."  or "... incompatible with
labeled statement ..." there? How would the full diagnostic for an
unreferenced global function (or data item) look like?

Jan