Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756503Ab0GMXtd (ORCPT ); Tue, 13 Jul 2010 19:49:33 -0400 Received: from claw.goop.org ([74.207.240.146]:60359 "EHLO claw.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751494Ab0GMXtc (ORCPT ); Tue, 13 Jul 2010 19:49:32 -0400 Message-ID: <4C3CFB8B.1090804@goop.org> Date: Tue, 13 Jul 2010 16:49:31 -0700 From: Jeremy Fitzhardinge User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.10) Gecko/20100621 Fedora/3.0.5-1.fc13 Lightning/1.0b2pre Thunderbird/3.0.5 MIME-Version: 1.0 To: "H. Peter Anvin" CC: Linus Torvalds , Peter Palfrader , Avi Kivity , Greg KH , linux-kernel@vger.kernel.org, stable@kernel.org, stable-review@kernel.org, akpm@linux-foundation.org, alan@lxorguk.ukuu.org.uk, Glauber Costa , Zachary Amsden , Marcelo Tosatti Subject: Re: [patch 134/149] x86, paravirt: Add a global synchronization point for pvclock References: <20100707124731.GJ15122@anguilla.noreply.org> <4C359D5A.1050906@redhat.com> <20100713102350.GW15122@anguilla.noreply.org> <4C3C68C8.4060409@redhat.com> <20100713141902.GB15122@anguilla.noreply.org> <4C3C8CE5.1080705@redhat.com> <20100713162207.GC15122@anguilla.noreply.org> <4C3C9589.4090602@redhat.com> <4C3C96EC.8060901@redhat.com> <4C3C9839.4090404@redhat.com> <20100713172526.GE15122@anguilla.noreply.org> <4C3CAE8F.10900@goop.org> <4C3CE560.5050701@zytor.com> In-Reply-To: <4C3CE560.5050701@zytor.com> X-Enigmail-Version: 1.0.1 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1767 Lines: 44 On 07/13/2010 03:14 PM, H. Peter Anvin wrote: > Actually, I believe volatile operations (including asm volatile) are > strictly ordered *with respect to other volatile operations*. The documentation makes no reference to that property; in fact it suggests it is outright not true: Note that even a volatile `asm' instruction can be moved relative to other code, including across jump instructions. For example, on many targets there is a system register which can be set to control the rounding mode of floating point operations. You might try setting it with a volatile `asm', like this PowerPC example: asm volatile("mtfsf 255,%0" : : "f" (fpenv)); sum = x + y; This will not work reliably, as the compiler may move the addition back before the volatile `asm'. To make it work you need to add an artificial dependency to the `asm' referencing a variable in the code you don't want moved, for example: asm volatile ("mtfsf 255,%1" : "=X"(sum): "f"(fpenv)); sum = x + y; Similarly, you can't expect a sequence of volatile `asm' instructions to remain perfectly consecutive. [...] An `asm' instruction without any output operands will be treated identically to a volatile `asm' instruction. > As such I > would think we'd want to keep the "memory" clobber here, to make it > strictly ordered with regards to *all* memory operations. > That would keep its overall effect consistent. 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/