From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932618Ab0E0Ugm (ORCPT ); Thu, 27 May 2010 16:36:42 -0400 Received: from mx1.redhat.com ([209.132.183.28]:22032 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759122Ab0E0Ugl (ORCPT ); Thu, 27 May 2010 16:36:41 -0400 Date: Thu, 27 May 2010 17:36:38 -0300 From: Glauber Costa To: Zachary Amsden Cc: Avi Kivity , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] Add Documentation/kvm/msr.txt Message-ID: <20100527203638.GH3445@mothafucka.localdomain> References: <1274897088-8587-1-git-send-email-glommer@redhat.com> <4BFE2A2F.50209@redhat.com> <20100527160248.GF3445@mothafucka.localdomain> <4BFED258.40501@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4BFED258.40501@redhat.com> X-ChuckNorris: True User-Agent: Jack Bauer Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 27, 2010 at 10:13:12AM -1000, Zachary Amsden wrote: > On 05/27/2010 06:02 AM, Glauber Costa wrote: > >On Thu, May 27, 2010 at 11:15:43AM +0300, Avi Kivity wrote: > >>On 05/26/2010 09:04 PM, Glauber Costa wrote: > >>>This patch adds a file that documents the usage of KVM-specific > >>>MSRs. > >>> > >>Looks good. A few comments: > >> > >>>+ > >>>+Custom MSR list > >>>+-------- > >>>+ > >>>+The current supported Custom MSR list is: > >>>+ > >>>+MSR_KVM_WALL_CLOCK: 0x11 > >>>+ > >>>+ data: physical address of a memory area. > >>Which must be in guest RAM (i.e., don't point it somewhere random > >>and expect the hypervisor to allocate it for you). > >> > >>Must be aligned to 4 bytes (we don't enforce it though). > >I don't see the reason for it. > > > >If this is a requirement, our own implementation > >is failing to meet it. > > It's so the atomic write actually is atomic. Which atomic write? This is the wallclock, we do no atomic writes for querying it. Not to confuse with system time (the other msr). > Stating a 4 -byte > alignment requirement prevents the wall clock from crossing a page > boundary. Yes, but why require it? reading the wallclock is not a hot path for anybody, is usually done just once, and crossing a page boundary here does not pose any correctness issue.