This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
Home Products Support Community News


RE: [Xen-devel] Re: TSC scaling and softtsc reprise, and PROPOSAL

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: RE: [Xen-devel] Re: TSC scaling and softtsc reprise, and PROPOSAL
From: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
Date: Wed, 5 Aug 2009 13:35:45 +0800
Accept-language: en-US
Acceptlanguage: en-US
Cc: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "Dong, Eddie" <eddie.dong@xxxxxxxxx>, Levon <levon@xxxxxxxxxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, John, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>
Delivery-date: Tue, 04 Aug 2009 22:37:03 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <4A78CCDD.9090205@xxxxxxxx>
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <C68F298A.104C0%keir.fraser@xxxxxxxxxxxxx> <4A78CCDD.9090205@xxxxxxxx>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
Thread-index: AcoVYI0nnN88UD3ETQuxEMRXQ3nhQgALTjSw
Thread-topic: [Xen-devel] Re: TSC scaling and softtsc reprise, and PROPOSAL
>From: Jeremy Fitzhardinge
>Sent: 2009年8月5日 8:06
>On 07/24/09 01:04, Keir Fraser wrote:
>> Okay, so the issue you are worried about is not specific to 
>Xen. So how is
>> native Linux tackling this, for example?
>Linux will use the tsc where possible, but regularly assesses its
>perceived accuracy and will move to a different clocksource if the tsc
>appears to the playing up.  I don't think it ever assumes the tsc is
>synced between CPU/cores.

It cares. See tsc_sync.c under x86 arch, where unsynced warps
mark tsc as unstable. 


>It allows rdtsc from usermode, but it is generally considered 
>to be very
>buggy and ill-defined behaviour.  It makes no attempt to make usermode
>rdtsc in any way meaningful.  The exception is the vgettimeofday
>vsyscall which does Xen-like timekeeping, in which it gets the tsc,cpu
>tuple atomically, then scales it with timing parameters from 
>the kernel.
>    J
>Xen-devel mailing list
Xen-devel mailing list