From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@qumranet.com>
Subject: Re: RFC: VMX: initialize TSC offset relative to vm creation time
Date: Sat, 13 Sep 2008 07:55:02 +0300
Message-ID: <48CB47A6.1050301@qumranet.com>
References: <20080910205842.GA12514@dmt.cnet>
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Cc: kvm-devel <kvm@vger.kernel.org>,
	"David S. Ahern" <daahern@cisco.com>,
	Chris Wright <chrisw@redhat.com>,
	Glauber de Oliveira Costa <gcosta@redhat.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Return-path: <kvm-owner@vger.kernel.org>
Received: from il.qumranet.com ([212.179.150.194]:21892 "EHLO il.qumranet.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1750851AbYIME53 (ORCPT <rfc822;kvm@vger.kernel.org>);
	Sat, 13 Sep 2008 00:57:29 -0400
In-Reply-To: <20080910205842.GA12514@dmt.cnet>
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

Marcelo Tosatti wrote:
> VMX initializes the TSC offset for each vcpu at different times, and
> also reinitializes it for vcpus other than 0 on APIC SIPI message.
>
> This bug causes the TSC's to appear unsynchronized in the guest, even if
> the host is good.
>
> Older Linux kernels don't handle the situation very well, so
> gettimeofday is likely to go backwards in time:
>
> http://www.mail-archive.com/kvm@vger.kernel.org/msg02955.html
> http://sourceforge.net/tracker/index.php?func=detail&aid=2025534&group_id=180599&atid=893831
>
> Fix it by initializating the offset of each vcpu relative to vm creation
> time, and moving it from vmx_vcpu_reset to vmx_vcpu_setup, out of the
> APIC MP init path.
>
>
>   

This is good in principle, but we need to detect if we're on a multiple
board host (or a host with unsynced tscs) and do something else in that
case.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.