Re: [RFC PATCH 04/20] compiler_types.h: Don't instrument __always_inline functions for coverage

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 28, 2026 at 02:51:40PM +0100, Peter Zijlstra wrote:
> On Wed, Oct 28, 2026 at 07:59:28AM -0400, Sasha Levin wrote:
> > noinstr code may only call other noinstr functions and __always_inline
> > functions. gcov and Clang's source-based code coverage insert their
> > instrumentation before inlining, so the instrumentation of an
> > __always_inline function ends up in every noinstr function it gets
> > inlined into. With Clang's source-based coverage on x86, 2500 of the
> > 3449 relocations in .noinstr.text and 333 of the 451 in .cpuidle.text
> > of vmlinux.o point to coverage counters or MC/DC bitmaps. GCC's gcov
> > does the same. Since these are plain memory updates rather than calls,
> > objtool's noinstr validation doesn't notice them.
> > 
> > KCOV doesn't have this problem, as it instruments the code after
> > inlining and checks the attributes of the function that the code ends
> > up in.
> > 
> > Make __always_inline imply __no_profile when a code coverage
> > measurement implementation is enabled, so that no noinstr code carries
> > coverage instrumentation. The cost is that __always_inline functions
> > don't show up in the coverage data at all, including at their callers
> > outside of noinstr code. For an x86_64 configuration built with Clang 22
> > and MC/DC, that removes 556 of 60788 functions (0.9%), 29261 of 1598305
> > regions (1.8%), 11067 of 591108 branches (1.9%) and 445 of 36561 MC/DC
> > conditions (1.2%) from the coverage data.
> 
> This is pretty vile. But also, since people were trying to use MC/DC for
> validation efforts and the like, how does this impact them? If I
> understand correctly their previous emails on the topic, this means they
> have to go manually audit every single __always_inline function.
> 
> Now; I understand this is a tool limitation and there really must not be
> instrumentation in noinstr code, since that can run before things like
> page-tables are complete (consider the PTI case). So this is the best
> option.
> 
> Still, I'm left wondering if these tools should be fixed. For instance,
> while the front-end emits this instrumentation, the backend could easily
> remove it again when it finds itself in the context of a noinstr
> function after inlining.

Ah, also, we have objtool's hack_noinstr :/ But sssh, don't tell Josh
about that.