linux-kernel - Re: [PATCH 00/22] add support for Clang LTO

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Message-ID: <20200706183933.GE9247@paulmck-ThinkPad-P72>
Date:   Mon, 6 Jul 2020 11:39:33 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marco Elver <elver@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-pci@...r.kernel.org,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>
Subject: Re: [PATCH 00/22] add support for Clang LTO

On Mon, Jul 06, 2020 at 08:29:26PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 06, 2020 at 09:26:33AM -0700, Paul E. McKenney wrote:
> 
> > And perhaps more constructively, we do need to prioritize address and data
> > dependencies over control dependencies.  For one thing, there are a lot
> > more address/data dependencies in existing code than there are control
> > dependencies, and (sadly, perhaps more importantly) there are a lot more
> > people who are convinced that address/data dependencies are important.
> 
> If they do not consider their Linux OS running correctly :-)

Many of them really do not care at all.  In fact, some would consider
Linux failing to run as an added bonus.

> > For another (admittedly more theoretical) thing, the OOTA scenarios
> > stemming from control dependencies are a lot less annoying than those
> > from address/data dependencies.
> > 
> > And address/data dependencies are as far as I know vulnerable to things
> > like conditional-move instructions that can cause problems for control
> > dependencies.
> > 
> > Nevertheless, yes, control dependencies also need attention.
> 
> Today I added one more \o/

Just make sure you continually check to make sure that compilers
don't break it, along with the others you have added.  ;-)

							Thanx, Paul