WARNING - OLD ARCHIVES

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/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-devel

Re: [Xen-devel] RE: rdtsc: correctness vs performance on Xen (and KVM?)

To: "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx>, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>
Subject: Re: [Xen-devel] RE: rdtsc: correctness vs performance on Xen (and KVM?)
From: "Jan Beulich" <JBeulich@xxxxxxxxxx>
Date: Wed, 02 Sep 2009 08:01:33 +0100
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>
Delivery-date: Wed, 02 Sep 2009 00:03:11 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <C6C34FE4.13AC4%keir.fraser@xxxxxxxxxxxxx>
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: <4A9D62460200007800012BA1@xxxxxxxxxxxxxxxxxx> <C6C34FE4.13AC4%keir.fraser@xxxxxxxxxxxxx>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
>>> Keir Fraser <keir.fraser@xxxxxxxxxxxxx> 01.09.09 23:25 >>>
>On 01/09/2009 17:04, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:
>
>>>>> Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> 01.09.09 17:56 >>>
>>> Can you think of any trick (that doesn't require the cost of a
>>> trap/hypercall) to allow an app to determine what pcpu
>>> it is running on?
>> 
>> Just like what is being used to allow apps to get the CPU number on native
>> kernels (or the vCPU one on Xen-ified ones): Have a GDT entry the limit of
>> which is the number you want, and have the app use the lsl instruction to
>> get at it.
>
>Yes, that's true. Xen could provide such a segment descriptor in its private
>area of the GDT. The issue then would be that, in a compound pvclock

And in fact there already is such a descriptor, just with DPL=0.

>operation spanning multiple machine instructions, the pCPU number revealed
>by the LSL instruction can be stale by the time it is used later in the
>compound operation.

Correct.

Jan



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel