linux-kernel - Re: [PATCH v2 12/12] x86/kvm/emulate: Avoid RET for fastops

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Message-ID: <20241116093954.GL22801@noisy.programming.kicks-ass.net>
Date: Sat, 16 Nov 2024 10:39:54 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: seanjc@...gle.com, pbonzini@...hat.com, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
	jthoughton@...gle.com
Subject: Re: [PATCH v2 12/12] x86/kvm/emulate: Avoid RET for fastops

On Fri, Nov 15, 2024 at 10:41:04AM -0800, Josh Poimboeuf wrote:
> On Mon, Nov 11, 2024 at 12:59:47PM +0100, Peter Zijlstra wrote:
> > Since there is only a single fastop() function, convert the FASTOP
> > stuff from CALL_NOSPEC+RET to JMP_NOSPEC+JMP, avoiding the return
> > thunks and all that jazz.
> > 
> > Specifically FASTOPs rely on the return thunk to preserve EFLAGS,
> > which not all of them can trivially do (call depth tracing suffers
> > here).
> > 
> > Objtool strenuously complains about this:
> > 
> >  - indirect call without a .rodata, fails to determine JUMP_TABLE,
> >    annotate
> >  - fastop functions fall through, exception
> >  - unreachable instruction after fastop_return, save/restore
> 
> This wording makes it sound like this patch triggers objtool warnings,
> which I'm guessing is not true?

Right, no, it did without the fixups. This was a (poorly worder) attempt
at explaining the reasons for the various annotations in the patch.