linux-kernel - Re: [RFC] Objtool toolchain proposal: -fannotate-{jump-table,noreturn}

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Message-ID: <alpine.LSU.2.20.2209141415340.8265@wotan.suse.de>
Date:   Wed, 14 Sep 2022 14:28:26 +0000 (UTC)
From:   Michael Matz <matz@...e.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Josh Poimboeuf <jpoimboe@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        linux-toolchains@...r.kernel.org,
        Indu Bhagat <indu.bhagat@...cle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, "Jose E. Marchesi" <jemarch@....org>,
        Miroslav Benes <mbenes@...e.cz>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Ard Biesheuvel <ardb@...nel.org>,
        Chen Zhongjin <chenzhongjin@...wei.com>,
        Sathvika Vasireddy <sv@...ux.ibm.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [RFC] Objtool toolchain proposal:
 -fannotate-{jump-table,noreturn}

Hello,

On Wed, 14 Sep 2022, Peter Zijlstra wrote:

> > Maybe this is semantics, but I wouldn't characterize objtool's existence
> > as being based on the mistrust of tools.  It's main motivation is to
> > fill in the toolchain's blind spots in asm and inline-asm, which exist
> > by design.
> 
> That and a fairly deep seated loathing for the regular CFI annotations
> and DWARF in general. Linus was fairly firm he didn't want anything to
> do with DWARF for in-kernel unwinding.

I was referring only to the check-stuff functionality of objtool, not to 
its other parts.  Altough, of course, "deep seated loathing" is a special 
form of mistrust as well ;-)

> That left us in a spot that we needed unwind information in a 'better'
> format than DWARF.
> 
> Objtool was born out of those contraints. ORC not needing the CFI
> annotations and ORC being *much* faster at unwiding and generation
> (debug builds are slow) were all good.

Don't mix DWARF debug info with DWARF-based unwinding info, the latter 
doesn't imply the former.  Out of interest: how does ORC get around the 
need for CFI annotations (or equivalents to restore registers) and what 
makes it fast?  I want faster unwinding for DWARF as well, when there's 
feature parity :-)  Maybe something can be learned for integration into 
dwarf-unwind.


Ciao,
Michael.