Re: [PATCH RFC 0/3] Static calls

[Ard Biesheuvel]

On 9 November 2018 at 08:28, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * 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.
>

I implemented the GCC plugin approach here for arm64

https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/log/?h=static-calls

That implements both the unoptimized and the optimized versions.

I do take your point about GCC and other compilers, but on arm64 we
don't have a lot of choice.

As far as I can tell, the GCC plugin is generic (i.e., it does not
rely on any ARM specific passes, but obviously, this requires a *lot*
of testing and validation to be taken seriously.

>> - 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