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

From: Michael Matz <matz@suse.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>,
	Borislav Petkov <bp@alien8.de>,
	linux-toolchains@vger.kernel.org,
	Indu Bhagat <indu.bhagat@oracle.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	linux-kernel@vger.kernel.org,
	"Jose E. Marchesi" <jemarch@gnu.org>,
	Miroslav Benes <mbenes@suse.cz>,
	Mark Rutland <mark.rutland@arm.com>,
	Will Deacon <will@kernel.org>,
	x86@kernel.org, linux-arm-kernel@lists.infradead.org,
	live-patching@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	Ard Biesheuvel <ardb@kernel.org>,
	Chen Zhongjin <chenzhongjin@huawei.com>,
	Sathvika Vasireddy <sv@linux.ibm.com>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	Mark Brown <broonie@kernel.org>
Subject: Re: [RFC] Objtool toolchain proposal: -fannotate-{jump-table,noreturn}
Date: Wed, 14 Sep 2022 14:28:26 +0000 (UTC)	[thread overview]
Message-ID: <alpine.LSU.2.20.2209141415340.8265@wotan.suse.de> (raw)
In-Reply-To: <YyHecBM8D0i1lRu8@hirez.programming.kicks-ass.net>

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.