From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com [148.163.156.1]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3ybkJt4cX0zDqlG for ; Tue, 14 Nov 2017 21:29:34 +1100 (AEDT) Received: from pps.filterd (m0098396.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id vAEATRR3015253 for ; Tue, 14 Nov 2017 05:29:31 -0500 Received: from e06smtp15.uk.ibm.com (e06smtp15.uk.ibm.com [195.75.94.111]) by mx0a-001b2d01.pphosted.com with ESMTP id 2e7upu1hj4-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Tue, 14 Nov 2017 05:29:30 -0500 Received: from localhost by e06smtp15.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 14 Nov 2017 10:29:27 -0000 Date: Tue, 14 Nov 2017 15:59:21 +0530 From: "Naveen N. Rao" Subject: Re: [PATCH v4 2/3] powerpc/modules: Don't try to restore r2 after a sibling call To: Kamalesh Babulal , Michael Ellerman Cc: Balbir Singh , Josh Poimboeuf , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, live-patching@vger.kernel.org References: <20171114092910.20399-1-kamalesh@linux.vnet.ibm.com> <20171114092910.20399-3-kamalesh@linux.vnet.ibm.com> In-Reply-To: <20171114092910.20399-3-kamalesh@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Message-Id: <1510654928.8xrjtkjm8m.naveen@linux.ibm.com> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Kamalesh Babulal wrote: > From: Josh Poimboeuf >=20 > When attempting to load a livepatch module, I got the following error: >=20 > module_64: patch_module: Expect noop after relocate, got 3c820000 >=20 > The error was triggered by the following code in > unregister_netdevice_queue(): >=20 > 14c: 00 00 00 48 b 14c > 14c: R_PPC64_REL24 net_set_todo > 150: 00 00 82 3c addis r4,r2,0 >=20 > GCC didn't insert a nop after the branch to net_set_todo() because it's > a sibling call, so it never returns. The nop isn't needed after the > branch in that case. >=20 > Signed-off-by: Josh Poimboeuf > Signed-off-by: Kamalesh Babulal > --- > arch/powerpc/kernel/module_64.c | 4 ++++ > 1 file changed, 4 insertions(+) >=20 > diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module= _64.c > index 39b01fd..9e5391f 100644 > --- a/arch/powerpc/kernel/module_64.c > +++ b/arch/powerpc/kernel/module_64.c > @@ -489,6 +489,10 @@ static int restore_r2(u32 *instruction, struct modul= e *me) > if (is_early_mcount_callsite(instruction - 1)) > return 1; >=20 > + /* Sibling calls don't return, so they don't need to restore r2 */ > + if (instruction[-1] =3D=3D PPC_INST_BRANCH) > + return 1; > + This looks quite fragile, unless we know for sure that gcc will _always_ emit this instruction form for sibling calls with relocations. As an alternative, does it make sense to do the following check instead? if ((instr_is_branch_iform(insn) || instr_is_branch_bform(insn)) && !(insn & 0x1)) - Naveen =