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: Saving/Restoring IA32_TSC_AUX MSR

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Fri, 11 Dec 2009 08:03:11 +0000
Cc: "Dugger, Donald D" <donald.d.dugger@xxxxxxxxx>
Delivery-date: Fri, 11 Dec 2009 00:03:37 -0800
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <ac1d995d-7658-4770-b2fe-14bf4f87d352@default>
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>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
Thread-index: Acp6BeJvxG9k2P+BT9C40QciKaIcFwAMn9NM
Thread-topic: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR
User-agent: Microsoft-Entourage/
On 11/12/2009 02:00, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> I expect that Keir will not support putting TSC_AUX
> in the context switch code unless it is absolutely
> necessary, as it is certainly expensive to read and
> write to TSC_AUX and this cost will add to every
> context switch of every VM even though very few will
> actually use rdtscp/TSC_AUX.

Well, you'd make it dependent on the guest using TSC_AUX, I suppose. I think
that's going to be pretty rare.

> So I think we need to decide first about approach (1),
> the full faithful implementation of rdtscp.

The question has to be: what win do we get for faithful virtualisation of
RDTSCP in a virtualised environment? Supporting CPU instructions just
because they're there is not a useful effort.

 -- Keir

Xen-devel mailing list