From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [patch 02/16] x86: pvclock: make sure rdtsc doesnt speculate out of region Date: Thu, 1 Nov 2012 18:56:11 -0200 Message-ID: <20121101205611.GB14888@amt.cnet> References: <20121031224656.417434866@redhat.com> <20121031224823.907464421@redhat.com> <20121101114807.GQ25650@redhat.com> <50927DFF.4000502@parallels.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Gleb Natapov , kvm@vger.kernel.org, johnstul@us.ibm.com, jeremy@goop.org, zamsden@gmail.com, avi@redhat.com, pbonzini@redhat.com To: Glauber Costa Return-path: Received: from mx1.redhat.com ([209.132.183.28]:58047 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759940Ab2KAVGg (ORCPT ); Thu, 1 Nov 2012 17:06:36 -0400 Content-Disposition: inline In-Reply-To: <50927DFF.4000502@parallels.com> Sender: kvm-owner@vger.kernel.org List-ID: On Thu, Nov 01, 2012 at 05:49:51PM +0400, Glauber Costa wrote: > On 11/01/2012 03:48 PM, Gleb Natapov wrote: > > On Wed, Oct 31, 2012 at 08:46:58PM -0200, Marcelo Tosatti wrote: > >> Originally from Jeremy Fitzhardinge. > >> > >> pvclock_get_time_values, which contains the memory barriers > >> will be removed by next patch. > >> > >> Signed-off-by: Marcelo Tosatti > >> > >> Index: vsyscall/arch/x86/kernel/pvclock.c > >> =================================================================== > >> --- vsyscall.orig/arch/x86/kernel/pvclock.c > >> +++ vsyscall/arch/x86/kernel/pvclock.c > >> @@ -97,10 +97,10 @@ cycle_t pvclock_clocksource_read(struct > >> > >> do { > >> version = pvclock_get_time_values(&shadow, src); > >> - barrier(); > >> + rdtsc_barrier(); > >> offset = pvclock_get_nsec_offset(&shadow); > >> ret = shadow.system_timestamp + offset; > >> - barrier(); > >> + rdtsc_barrier(); > >> } while (version != src->version); > >> > >> if ((valid_flags & PVCLOCK_TSC_STABLE_BIT) && > >> > > On a guest without SSE2 rdtsc_barrier() will be nop while rmb() will > > be "lock; addl $0,0(%%esp)". I doubt pvclock will work correctly either > > way though. > > > > -- > > Gleb. > > > Actually it shouldn't matter for KVM, since the page is only updated by > the vcpu, and the guest is never running while it happens. If Jeremy is > fine with this, so should I. 17.13 TIME-STAMP COUNTER "The RDTSC instruction is not serializing or ordered with other instructions. It does not necessarily wait until all previous instructions have been executed before reading the counter. Similarly, subsequent instructions may begin execution before the RDTSC instruction operation is performed." Both instructions are TSC barriers.