Re: [PATCH RFC 0/3] Static calls

[Ingo Molnar]

* Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:

> These patches are related to two similar patch sets from Ard and Steve:
> 
> - https://lkml.kernel.org/r/20181005081333.15018-1-ard.biesheuvel@xxxxxxxxxx
> - https://lkml.kernel.org/r/20181006015110.653946300@xxxxxxxxxxx
> 
> 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