From mboxrd@z Thu Jan 1 00:00:00 1970 From: Zachary Amsden Subject: Re: [PATCH] Add Documentation/kvm/msr.txt Date: Thu, 27 May 2010 11:02:35 -1000 Message-ID: <4BFEDDEB.9020700@redhat.com> References: <1274897088-8587-1-git-send-email-glommer@redhat.com> <4BFE2A2F.50209@redhat.com> <20100527160248.GF3445@mothafucka.localdomain> <4BFED258.40501@redhat.com> <20100527203638.GH3445@mothafucka.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Avi Kivity , kvm@vger.kernel.org, linux-kernel@vger.kernel.org To: Glauber Costa Return-path: In-Reply-To: <20100527203638.GH3445@mothafucka.localdomain> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 05/27/2010 10:36 AM, Glauber Costa wrote: > 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. > Little-endian non-atomic page crossing writes will write the small part of the wallclock first, so another CPU may observe the following wallclock sequence: 0x01ff .. 0x0100 .. 0x0200 Big-endian writes also have similar failure: 0x01ff .. 0x02ff .. 0x0200 This won't happen if there is a single instruction write of the wall clock word.