linux-kernel - Re: [PATCH 3/6] x86/kvm/emulate: Avoid RET for fastops

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Message-ID: <7vfbchsyhlsvdl4hszdtmapdghw32nrj2qd652f3pjzg3yb6vn@po3bsa54b6ta>
Date: Mon, 14 Apr 2025 15:36:50 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org, kys@...rosoft.com, haiyangz@...rosoft.com, 
	wei.liu@...nel.org, decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com, 
	bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com, 
	pawan.kumar.gupta@...ux.intel.com, seanjc@...gle.com, pbonzini@...hat.com, ardb@...nel.org, 
	kees@...nel.org, Arnd Bergmann <arnd@...db.de>, gregkh@...uxfoundation.org, 
	linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	linux-efi@...r.kernel.org, samitolvanen@...gle.com, ojeda@...nel.org
Subject: Re: [PATCH 3/6] x86/kvm/emulate: Avoid RET for fastops

On Mon, Apr 14, 2025 at 01:11:43PM +0200, 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 things, therefore fix up the
> various problems:
> 
>  - indirect call without a .rodata, fails to determine JUMP_TABLE,
>    add an annotation for this.
>  - fastop functions fall through, create an exception for this case
>  - unreachable instruction after fastop_return, save/restore

I think this breaks unwinding.  Each of the individual fastops inherits
fastop()'s stack but the ORC doesn't reflect that.

Should they just be moved to a proper .S file?

-- 
Josh