Lore thread start for newly cc'ed ML readers: https://lore.kernel.org/lkml/7fad83ecde03540e65677959034315f8fbb3755e.1649434832.git.jpoimboe@xxxxxxxxxx/ On Fri, Apr 8, 2022 at 12:14 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > On Sat, Apr 09, 2022 at 03:29:21AM +0900, Masahiro Yamada wrote: > > Is [2] caused by dead code that was not optimized out > > due to the unusual inlining decisions by the compiler ? > > The complaint is due to SMAP validation; objtool will scream if there's > a CALL in between STAC/CLAC. The thinking is that since they open a > security window, we want tight code between them. We also very much > don't want tracing and other funnies to happen there. As such, any CALL > is dis-allowed. Just indirect calls, which might be manipulated, or static calls, too? > > This weird option is having us upgrade quite a few 'inline' to > '__always_inline'. As is, the assumption that __init functions only call other __init functions or __always_inline is a brittle house of cards that leads to a "what color is your function" [0] scenario, and leads to code that happens to not emit warnings for compiler X (or compiler X version Y). There's also curious exceptions in modpost that look like memory leaks to me. We already have such toolchain portability issues for different toolchains and different configs; warnings from section mismatches, and objtool STAC/CLAC checks. I feel that Josh's patch would sweep more of those under the rug, so I'm not in favor of it, but could be convinced otherwise. TBH, I kind of think that we could use a C extension to permit __attribute__((always_inline)) to additionally be a statement attribute, rather than just a function attribute because of cases like this; we need the flexibility to make one call site __always_inline without necessarily forcing ALL callsites to be __always_inline'd. void y (void); void x (void) { __attribute__((always_inline)) y(); }; (This is already expressable in LLVM IR; not (yet) in C. I'm not sure yet _why_ this was added to LLVM; whether a different language front end can express this, if C can and I'm mistaken, or whether it's only used for optimizations). I think that would give developers maximal flexibility to defer as much to the compiler's inlining decisions when they don't care, and express precisely what they need when they do [care]. [0] https://journal.stuffwithstuff.com/2015/02/01/what-color-is-your-function/ -- Thanks, ~Nick Desaulniers
