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Date:   Tue, 28 Jan 2020 10:28:07 +0100 (CET)
From:   Miroslav Benes <mbenes@suse.cz>
To:     Josh Poimboeuf <jpoimboe@redhat.com>
cc:     Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Joe Lawrence <joe.lawrence@redhat.com>,
        Jessica Yu <jeyu@kernel.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org, mhiramat@kernel.org,
        bristot@redhat.com, jbaron@akamai.com,
        torvalds@linux-foundation.org, tglx@linutronix.de,
        mingo@kernel.org, namit@vmware.com, hpa@zytor.com, luto@kernel.org,
        ard.biesheuvel@linaro.org, live-patching@vger.kernel.org,
        Randy Dunlap <rdunlap@infradead.org>
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()
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On Wed, 22 Jan 2020, Josh Poimboeuf wrote:

> On Wed, Jan 22, 2020 at 11:09:56AM +0100, Miroslav Benes wrote:
> > 
> > > > > At this point, I only see downsides of -flive-patching, at least until
> > > > > we actually have real upstream code which needs it.
> > > > 
> > > > Can you explain this? The option makes GCC to avoid optimizations which 
> > > > are difficult to detect and would make live patching unsafe. I consider it 
> > > > useful as it is, so if you shared the other downsides and what you meant 
> > > > by real upstream code, we could discuss it.
> > > 
> > > Only SLES needs it right?  Why inflict it on other livepatch users?  By
> > > "real upstream code" I mean there's no (documented) way to create live
> > > patches using the method which relies on this flag.  So I don't see any
> > > upstream benefits for having it enabled.
> > 
> > I'd put it differently. SLES and upstream need it, RHEL does not need it. 
> > Or anyone using kpatch-build.
> 
> I'm confused about why you think upstream needs it.
> 
> Is all the tooling available somewhere?  Is there documentation
> available which describes how to build patches using that method from
> start to finish?  Are there actual users other than SUSE?
> 
> BTW, kpatch-build has a *lot* of users other than RHEL.  All its tooling
> and documentation are available on Github.
> 
> > It is perfectly fine to prepare live patches just from the source code
> > using upstream live patching infrastructure. 
> 
> Do you mean the dangerous method used by the livepatch sample code which
> completely ignores interprocedural optimizations?  I wouldn't call that
> perfectly fine.
> 
> > After all, SLES is nothing else than upstream here. We were creating live 
> > patches manually for quite a long time and only recently we have been 
> > using Nicolai's klp-ccp automation (https://github.com/SUSE/klp-ccp).
> > 
> > So, everyone using upstream directly relies on the flag, which seems to be 
> > a clear benefit to me. Reverting the patch would be a step back.
> 
> Who exactly is "everyone using upstream"?
> 
> >From what I can tell, kpatch-build is the only known way (to those
> outside of SUSE) to make safe patches for an upstream kernel.  And it
> doesn't need this flag and the problems associated with it: performance,
> LTO incompatibility, clang incompatibility (I think?), the GCC dead code
> issue.

I don't think we have something special at SUSE not generally available...

...and I don't think it is really important to discuss that and replying 
to the above, because there is a legitimate use case which relies on the 
flag. We decided to support different use cases right at the beginning.

I understand it currently complicates things for objtool, but objtool is 
sensitive to GCC code generation by definition. "Issues" appear with every 
new GCC version. I see no difference here and luckily it is not so 
difficult to fix it.

I am happy to help with acting on those objtool warning reports you 
mentioned in the other email. Just Cc me where appropriate. We will take a 
look.

Regards
Miroslav