* Josh Poimboeuf <jpoim...@redhat.com> wrote:
> These patches are related to two similar patch sets from Ard and Steve: > > - https://lkml.kernel.org/r/20181005081333.15018-1-ard.biesheu...@linaro.org > - https://lkml.kernel.org/r/20181006015110.653946...@goodmis.org > > The code is also heavily inspired by the jump label code, as some of the > concepts are very similar. > > There are three separate implementations, depending on what the arch > supports: > > 1) CONFIG_HAVE_STATIC_CALL_OPTIMIZED: patched call sites - requires > objtool and a small amount of arch code > > 2) CONFIG_HAVE_STATIC_CALL_UNOPTIMIZED: patched trampolines - requires > a small amount of arch code > > 3) If no arch support, fall back to regular function pointers > > > TODO: > > - I'm not sure about the objtool approach. Objtool is (currently) > x86-64 only, which means we have to use the "unoptimized" version > everywhere else. I may experiment with a GCC plugin instead. I'd prefer the objtool approach. It's a pretty reliable first-principles approach while GCC plugin would have to be replicated for Clang and any other compilers, etc. > - Does this feature have much value without retpolines? If not, should > we make it depend on retpolines somehow? Paravirt patching, as you mention in your later reply? > - Find some actual users of the interfaces (tracepoints? crypto?) I'd be very happy with a demonstrated paravirt optimization already - i.e. seeing the before/after effect on the vmlinux with an x86 distro config. All major Linux distributions enable CONFIG_PARAVIRT=y and CONFIG_PARAVIRT_XXL=y on x86 at the moment, so optimizing it away as much as possible in the 99.999% cases where it's not used is a primary concern. All other usecases are bonus, but it would certainly be interesting to investigate the impact of using these APIs for tracing: that too is a feature enabled everywhere but utilized only by a small fraction of Linux users - so literally every single cycle or instruction saved or hot-path shortened is a major win. Thanks, Ingo