From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757577AbbFQAiD (ORCPT ); Tue, 16 Jun 2015 20:38:03 -0400 Received: from mail.kernel.org ([198.145.29.136]:54499 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757491AbbFQAhc (ORCPT ); Tue, 16 Jun 2015 20:37:32 -0400 From: Andy Lutomirski To: x86@kernel.org Cc: Borislav Petkov , Peter Zijlstra , John Stultz , linux-kernel@vger.kernel.org, Len Brown , Huang Rui , Denys Vlasenko , kvm@vger.kernel.org, Ralf Baechle , Andy Lutomirski , Paolo Bonzini , Radim Krcmar , Marcelo Tosatti Subject: [PATCH v3 17/18] x86/kvm/tsc: Drop extra barrier and use rdtsc_ordered in kvmclock Date: Tue, 16 Jun 2015 17:36:05 -0700 Message-Id: <678981cc4761fb38a793c217c9cac42503cf3719.1434501121.git.luto@kernel.org> X-Mailer: git-send-email 2.4.2 In-Reply-To: References: In-Reply-To: References: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org __pvclock_read_cycles had an unnecessary barrier. Get rid of that barrier and clean up the code by just using rdtsc_ordered(). Cc: Paolo Bonzini Cc: Radim Krcmar Cc: Marcelo Tosatti Cc: kvm@vger.kernel.org Signed-off-by: Andy Lutomirski --- I'm hoping to get an ack for this to go in through -tip. (Arguably I'm the maintainer of this code given how it's used, but I should still ask for an ack.) arch/x86/include/asm/pvclock.h | 21 ++++++++++++--------- 1 file changed, 12 insertions(+), 9 deletions(-) diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h index 6084bce345fc..cf2329ca4812 100644 --- a/arch/x86/include/asm/pvclock.h +++ b/arch/x86/include/asm/pvclock.h @@ -62,7 +62,18 @@ static inline u64 pvclock_scale_delta(u64 delta, u32 mul_frac, int shift) static __always_inline u64 pvclock_get_nsec_offset(const struct pvclock_vcpu_time_info *src) { - u64 delta = rdtsc() - src->tsc_timestamp; + /* + * Note: emulated platforms which do not advertise SSE2 support + * break rdtsc_ordered, resulting in kvmclock not using the + * necessary RDTSC barriers. Without barriers, it is possible + * that RDTSC instruction is executed before prior loads, + * resulting in violation of monotonicity. + * + * On an SMP guest without SSE2, it's unclear how anything is + * supposed to work correctly, though -- memory fences + * (e.g. smp_mb) are important for more than just timing. + */ + u64 delta = rdtsc_ordered() - src->tsc_timestamp; return pvclock_scale_delta(delta, src->tsc_to_system_mul, src->tsc_shift); } @@ -76,17 +87,9 @@ unsigned __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src, u8 ret_flags; version = src->version; - /* Note: emulated platforms which do not advertise SSE2 support - * result in kvmclock not using the necessary RDTSC barriers. - * Without barriers, it is possible that RDTSC instruction reads from - * the time stamp counter outside rdtsc_barrier protected section - * below, resulting in violation of monotonicity. - */ - rdtsc_barrier(); offset = pvclock_get_nsec_offset(src); ret = src->system_time + offset; ret_flags = src->flags; - rdtsc_barrier(); *cycles = ret; *flags = ret_flags; -- 2.4.2