GCC options for kernel live-patching (Was: Add a new option to control inlining only on static functions)

[Qing Zhao]

Thanks, Martin, for all these interesting information.

Looks like that a more general option to help live-patching is needed. 

I will do a little more study on this direction.

Qing
> On Sep 27, 2018, at 7:19 AM, Martin Jambor <mjambor@suse.cz> wrote:
> 
> Hi,
> 
> (this message is a part of the thread originating with
> https://gcc.gnu.org/ml/gcc-patches/2018-09/msg01018.html)
> 
> On Thu, Sep 27 2018, Jan Hubicka wrote:
>>>> If you make this to be INTERPOSABLE (which means it can be replaced by different
>>>> implementation by linker and that is probably what we want for live patching)
>>>> then also inliner, ipa-sra and other optimization will give up on these.
>>> 
>>> do you suggest that to set the global function as AVAIL_INTERPOSABLE when -finline-only-static 
>>> is present? then we should avoid all issues?
>> 
>> It seems to be reasonable direction I think, because it is what really happens
>> (well AVAIL_INTERPOSABLE still does not assume that the interposition will
>> happen at runtime, but it is an approximation and we may introduce something like
>> AVAIL_RUNTIME_INTERPOSABLE if there is need for better difference).
>> I wonder if -finline-only-static is good name for the flag though, because it
>> does a lot more than that.  Maybe something like -flive-patching?
>> How much is this all tied to one particular implementation of the feature?
> 
> We have just had a quick discussion with two upstream maintainers of
> Linux kernel live-patching about this and the key points were:
> 
> 1. SUSE live-patch creators (and I assume all that use the upstream
>   live-patching method) use Martin Liska's (somewhat under-documented)
>   -fdump-ipa-clones option and a utility he wrote
>   (https://github.com/marxin/kgraft-analysis-tool) to deal with all
>   kinds of inlining, IPA-CP and generally all IPA optimizations that
>   internally create a clone.  The tool tells them what happened and
>   also lists all callers that need to be live-patched.
> 
> 2. However, there is growing concern about other IPA analyses that do
>   not create a clone but still affect code generation in other
>   functions.  Kernel developers have identified and disabled IPA-RA but
>   there is more of them such as IPA-modref analysis, stack alignment
>   propagation and possibly quite a few others which extract information
>   from one function and use it a caller or perhaps even some
>   almost-unrelated functions (such as detection of read-only and
>   write-only static global variables).
> 
>   The kernel live-patching community would welcome if GCC had an option
>   that could disable all such optimizations/analyses for which it
>   cannot provide a list of all affected functions (i.e. which ones need
>   to be live-patched if a particular function is).
> 
>   I assume this is orthogonal to the proposed -finline-only-static
>   option, but the above approach seems superior in all respects.
> 
> 3. The community would also like to be involved in these discussions,
>   and therefore I am adding live-patching@vger.kernel.org to CC.  On a
>   related note, they will also have a live-patching mini-summit at the
>   Linux Plumbers conference in Vancouver in November where they plan to
>   discuss what they would like GCC to provide.
> 
> Thanks,
> 
> Martin
> 
> ...
>>>>> 
>>>>>> For example comdat that was cloned by IPA-SRA. See can_be_local_p and
>>>>>> comdat_can_be_unshared_p predicates.  Similar problem happens to clones created
>>>>>> by ipa-cp.
>>>>>> 
>>>>>> I guess we want to disable localization and cloning in this case as well.
>>>>>> I wonder what else.
>>>>> 
>>>>> Yes, I think we should make -finline-only-static incompatible with cloning and tree-sra too.
>>>>> 
>>>>> Qing
>>>>>> 
>>>>>> Honza
>>>