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

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Message-ID: <20241115184104.pud5l26v5vx3vcbp@jpoimboe>
Date: Fri, 15 Nov 2024 10:41:04 -0800
From: Josh Poimboeuf <jpoimboe@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
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 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?

-- 
Josh