Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933061AbZJLTOS (ORCPT ); Mon, 12 Oct 2009 15:14:18 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932718AbZJLTOR (ORCPT ); Mon, 12 Oct 2009 15:14:17 -0400 Received: from claw.goop.org ([74.207.240.146]:59648 "EHLO claw.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932643AbZJLTOQ (ORCPT ); Mon, 12 Oct 2009 15:14:16 -0400 Message-ID: <4AD37FE3.1010002@goop.org> Date: Mon, 12 Oct 2009 12:13:39 -0700 From: Jeremy Fitzhardinge User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.4pre) Gecko/20090922 Fedora/3.0-2.7.b4.fc11 Lightning/1.0pre Thunderbird/3.0b4 MIME-Version: 1.0 To: Avi Kivity CC: Dan Magenheimer , Jeremy Fitzhardinge , kurt.hackel@oracle.com, the arch/x86 maintainers , Linux Kernel Mailing List , Glauber de Oliveira Costa , Xen-devel , Keir Fraser , Zach Brown , Chris Mason Subject: Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation References: <1254790211-15416-1-git-send-email-jeremy.fitzhardinge@citrix.com> <1254790211-15416-4-git-send-email-jeremy.fitzhardinge@citrix.com> <4ACB0833.2050203@redhat.com> <4ACB9074.1000804@goop.org> <4ACC6C9C.7080707@redhat.com> <4ACFD43E.6000506@goop.org> <4AD0CDFB.9030704@redhat.com> <4AD3738B.6050200@goop.org> <4AD375A5.8050205@redhat.com> In-Reply-To: <4AD375A5.8050205@redhat.com> X-Enigmail-Version: 0.97a Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1809 Lines: 43 On 10/12/09 11:29, Avi Kivity wrote: > Good catch. Doesn't that invalidate rdtscp based vgettimeofday on > non-virt as well (assuming p == cpu)? The tsc clocksource assumes the tsc is (mostly?) synced; it doesn't use rdtscp or make any attempt at per-cpu corrections. >> I suppose that works if you assume that: >> >> 1. every task->vcpu migration is associated with a hv/guest context >> switch, and >> 2. every hv/guest context switch is a write barrier >> >> I guess 2 is a given, but I can at least imagine cases where 1 might not >> be true. Maybe. It all seems very subtle. >> > > What is 1 exactly? task switching to another vcpu? that doesn't > incur hypervisor involvement. vcpu moving to another cpu? That does. Aie... OK. So no barrier is required for a task double migration on vcpus, because it ends up on the same pcpu and the ordering is local; if there's a vcpu migration to a new pcpu in there too, then we always expect a barrier. >> And I don't really see a gain. You avoid maintaining a second version >> number, but at the cost of two rdtscps. In my measurements, the whole >> vsyscall takes around 100ns to run, and a single rdtsc takes about 30, >> so 30% of total. Unlike rdtsc, rdtscp is documented as being ordered in >> the instruction stream, and so will take at least as long; two of them >> will completely blow the vsyscall execution time. >> > > I agree, let's stick with the rdtscpless implementation. OK, I'll use PeterZ's hint to try and find a more complete set of migration points. J -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/