Messages in this thread | | | Date | Mon, 14 Jun 2021 11:31:35 -0700 | From | Fangrui Song <> | Subject | Re: [PATCH] gcov,x86: Mark GCOV broken for x86 |
| |
> On Mon, Jun 14, 2021 at 9:20 AM Peter Zijlstra <peterz@infradead.org> wrote: > > > > On Mon, Jun 14, 2021 at 09:05:04AM -0700, Nick Desaulniers wrote: > > > On Mon, Jun 14, 2021 at 3:17 AM Peter Zijlstra <peterz@infradead.org> wrote: > > > > > > > > > > > > As recently discovered, there is no function attribute to disable the > > > > -fprofile-generate instrumentation. As such, GCOV is fundamentally > > > > incompatible with architectures that rely on 'noinstr' for correctness. > > > > > > Is there context for comment, or is this patch meant as a joke? > > > > Only if you think recursion in exception entry code is funny. > > > > noinstr *MUST* disable any and all compiler generated instrumentation, > > currently it that isn't the case for -fprofile-gnerate, nor > > -fprofile-arc. > > > > Look for all the fun we had with KCOV back then. Luckily KCOV > > instrumentation was trivial to patch out using objtool, so that's what > > x86 is currently doing. > > > > Luckily both compilers grew a __no_sanitize_coverage recently and we no > > longer have to rely on objtool fixing up the compiler output for much > > longer. > > > > https://lkml.kernel.org/r/20210527194448.3470080-1-elver@google.com > > > > Now all we need is one more such attribute to kill -fprofile-* stuff. > > __attribute__((no_instrument_function)) is already wired to not emit > calls to mcount()/fentry(). I think extending it to also apply to > coverage (-fprofile-arcs) and instrumentation based profiling > (-fprofile-generate) is reasonable.
__attribute__((no_instrument_function)) seems specific to -finstrument-functions. Somehow -pg uses it as well. The name may not be generic, so it may be odd to exclude various instrumentations (there are a ton) under this generic attribute.
I'd like to understand the definition of notrace and noinstr.
With value profiling disabled, clang -fprofile-generate/gcc -fprofile-arcs don't add function calls. They just increment a counter in a writable section. Why isn't that allowed for noinstr functions?
I can understand why -fpatchable-function-entry= is excluded: -fpatchable-function-entry= causes the section __patchable_function_entries and the kernel may change the nops into call instructions. And a function call may not be suitable for certain functions. But I don't understand why incrementing a counter should be disallowed.
| |