From:   "Guilherme G. Piccoli" <gpiccoli@igalia.com>
To:     linux-hyperv@vger.kernel.org, x86@kernel.org
Cc:     linux-kernel@vger.kernel.org, decui@microsoft.com,
        haiyangz@microsoft.com, kys@microsoft.com, wei.liu@kernel.org,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, hpa@zytor.com,
        thomas.lendacky@amd.com, jpoimboe@kernel.org, peterz@infradead.org,
        kernel-dev@igalia.com, kernel@gpiccoli.net,
        "Guilherme G. Piccoli" <gpiccoli@igalia.com>,
        Arnd Bergmann <arnd@arndb.de>
Subject: [PATCH] x86/hyperv: Mark hv_ghcb_terminate() as noreturn
Date:   Fri, 10 Mar 2023 11:02:52 -0300
Message-Id: <20230310140251.1159036-1-gpiccoli@igalia.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

Annotate the function prototype as noreturn to prevent objtool
warnings like:

vmlinux.o: warning: objtool: hyperv_init+0x55c: unreachable instruction

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@arndb.de>
Link: https://lore.kernel.org/r/9698eff1-9680-4f0a-94de-590eaa923e94@app.fastmail.com/
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
---


Hey folks, after getting the warning myself a quick search led me to Arnd's
thorough report - investigating a bit, this seems to be the proper solution.

Notice I didn't add the function to objtool's static list, seems this is
unnecessary in this case - lemme know otherwise!
Thanks in advance for reviews,


Guilherme


 arch/x86/include/asm/mshyperv.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index 4c4c0ec3b62e..09c26e658bcc 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -212,7 +212,7 @@ int hv_set_mem_host_visibility(unsigned long addr, int numpages, bool visible);
 void hv_ghcb_msr_write(u64 msr, u64 value);
 void hv_ghcb_msr_read(u64 msr, u64 *value);
 bool hv_ghcb_negotiate_protocol(void);
-void hv_ghcb_terminate(unsigned int set, unsigned int reason);
+void __noreturn hv_ghcb_terminate(unsigned int set, unsigned int reason);
 #else
 static inline void hv_ghcb_msr_write(u64 msr, u64 value) {}
 static inline void hv_ghcb_msr_read(u64 msr, u64 *value) {}
-- 
2.39.2