Messages in this thread | | | Date | Fri, 9 Nov 2018 08:28:11 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH RFC 0/3] Static calls |
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* Josh Poimboeuf <jpoimboe@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.biesheuvel@linaro.org > - https://lkml.kernel.org/r/20181006015110.653946300@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
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