From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755472AbbBSBCw (ORCPT ); Wed, 18 Feb 2015 20:02:52 -0500 Received: from youngberry.canonical.com ([91.189.89.112]:35387 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753612AbbBSAd7 (ORCPT ); Wed, 18 Feb 2015 19:33:59 -0500 From: Kamal Mostafa To: linux-kernel@vger.kernel.org, stable@vger.kernel.org, kernel-team@lists.ubuntu.com Cc: Marcelo Tosatti , Andy Lutomirski , Luis Henriques , Kamal Mostafa Subject: [PATCH 3.13.y-ckt 021/103] x86, vdso: Use asm volatile in __getcpu Date: Wed, 18 Feb 2015 16:31:48 -0800 Message-Id: <1424305990-17983-22-git-send-email-kamal@canonical.com> X-Mailer: git-send-email 1.9.1 In-Reply-To: <1424305990-17983-1-git-send-email-kamal@canonical.com> References: <1424305990-17983-1-git-send-email-kamal@canonical.com> X-Extended-Stable: 3.13 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.13.11-ckt16 -stable review patch. If anyone has any objections, please let me know. ------------------ From: Andy Lutomirski commit 1ddf0b1b11aa8a90cef6706e935fc31c75c406ba upstream. In Linux 3.18 and below, GCC hoists the lsl instructions in the pvclock code all the way to the beginning of __vdso_clock_gettime, slowing the non-paravirt case significantly. For unknown reasons, presumably related to the removal of a branch, the performance issue is gone as of e76b027e6408 x86,vdso: Use LSL unconditionally for vgetcpu but I don't trust GCC enough to expect the problem to stay fixed. There should be no correctness issue, because the __getcpu calls in __vdso_vlock_gettime were never necessary in the first place. Note to stable maintainers: In 3.18 and below, depending on configuration, gcc 4.9.2 generates code like this: 9c3: 44 0f 03 e8 lsl %ax,%r13d 9c7: 45 89 eb mov %r13d,%r11d 9ca: 0f 03 d8 lsl %ax,%ebx This patch won't apply as is to any released kernel, but I'll send a trivial backported version if needed. [ Backported by Andy Lutomirski. Should apply to all affected versions. This fixes a functionality bug as well as a performance bug: buggy kernels can infinite loop in __vdso_clock_gettime on affected compilers. See, for exammple: https://bugzilla.redhat.com/show_bug.cgi?id=1178975 ] Fixes: 51c19b4f5927 x86: vdso: pvclock gettime support Cc: Marcelo Tosatti Acked-by: Paolo Bonzini Signed-off-by: Andy Lutomirski [ luis: backported to 3.16: used Andy's backport for stable kernels ] Signed-off-by: Luis Henriques Signed-off-by: Kamal Mostafa --- arch/x86/include/asm/vsyscall.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/x86/include/asm/vsyscall.h b/arch/x86/include/asm/vsyscall.h index 2a46ca7..2874be9 100644 --- a/arch/x86/include/asm/vsyscall.h +++ b/arch/x86/include/asm/vsyscall.h @@ -34,7 +34,7 @@ static inline unsigned int __getcpu(void) native_read_tscp(&p); } else { /* Load per CPU data from GDT */ - asm("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG)); + asm volatile ("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG)); } return p; -- 1.9.1