linux-kernel - Re: [PATCH v3] x86/hyperv: Mark hv_ghcb

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Message-ID: <20230317162434.dpby4idecp7a6hrg@treble>
Date:   Fri, 17 Mar 2023 09:24:34 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>
Cc:     linux-hyperv@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, decui@...rosoft.com,
        haiyangz@...rosoft.com, kys@...rosoft.com, wei.liu@...nel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com,
        thomas.lendacky@....com, peterz@...radead.org,
        kernel-dev@...lia.com, kernel@...ccoli.net,
        Arnd Bergmann <arnd@...db.de>,
        Michael Kelley <mikelley@...rosoft.com>
Subject: Re: [PATCH v3] x86/hyperv: Mark hv_ghcb_terminate() as noreturn

On Fri, Mar 17, 2023 at 01:05:46PM -0300, Guilherme G. Piccoli wrote:
> Annotate the function prototype and definition as noreturn to prevent
> objtool warnings like:
> 
> vmlinux.o: warning: objtool: hyperv_init+0x55c: unreachable instruction
> 
> Also, as per Josh's suggestion, add it to the global_noreturns list.
> As a comparison, an objdump output without the annotation:
> 
> [...]
> 1b63:  mov    $0x1,%esi
> 1b68:  xor    %edi,%edi
> 1b6a:  callq  ffffffff8102f680 <hv_ghcb_terminate>
> 1b6f:  jmpq   ffffffff82f217ec <hyperv_init+0x9c> # unreachable
> 1b74:  cmpq   $0xffffffffffffffff,-0x702a24(%rip)
> [...]
> 
> Now, after adding the __noreturn to the function prototype:
> 
> [...]
> 17df:  callq  ffffffff8102f6d0 <hv_ghcb_negotiate_protocol>
> 17e4:  test   %al,%al
> 17e6:  je     ffffffff82f21bb9 <hyperv_init+0x469>
> [...]  <many insns>
> 1bb9:  mov    $0x1,%esi
> 1bbe:  xor    %edi,%edi
> 1bc0:  callq  ffffffff8102f680 <hv_ghcb_terminate>
> 1bc5:  nopw   %cs:0x0(%rax,%rax,1) # end of function
> 
> Reported-by: Arnd Bergmann <arnd@...db.de>
> Link: https://lore.kernel.org/r/9698eff1-9680-4f0a-94de-590eaa923e94@app.fastmail.com/
> Cc: Josh Poimboeuf <jpoimboe@...nel.org>
> Cc: Michael Kelley <mikelley@...rosoft.com>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@...lia.com>

Looks good to me.  I've got some other noreturn fixes pending, so I can
add this patch to the pile unless somebody else wants to take it.

-- 
Josh