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] [PATCH 05/12] xen/pvclock: add monotonicity check

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: RE: [Xen-devel] [PATCH 05/12] xen/pvclock: add monotonicity check
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Thu, 15 Oct 2009 06:27:24 -0700 (PDT)
Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, kurt.hackel@xxxxxxxxxx, arch/x86 maintainers <x86@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Glauber de Oliveira Costa <gcosta@xxxxxxxxxx>, Avi Kivity <avi@xxxxxxxxxx>, chris.mason@xxxxxxxxxx
Delivery-date: Thu, 15 Oct 2009 06:35:35 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <4AD6B1F0.6030904@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>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> On 10/14/09 20:26, Dan Magenheimer wrote:
> > As long as we are going through the trouble of making
> > this monotonic, shouldn't it be monotonically increasing
> > (rather than just monotonically non-decreasing)?  The
> > rdtsc instruction and any suitably high-precision
> > hardware timer will never return the same value
> > on subsequent uses so this might be a reasonable
> > precedent to obey.  E.g.
> >
> > +   return ret > xen_clocksource.cycle_last ?
> > +           ret : ++xen_clocksource.cycle_last;
> 
> No, cycle_last isn't updated on every read, only on timer ticks.  This
> test doesn't seem to be intended to make sure that every
> clocksource_read is globally monotonic, but just to avoid 
> some boundary
> conditions in the timer interrupt.  I just copied it directly from
> read_tsc().

I understand but you are now essentially emulating a
reliable platform timer with a potentially unreliable
(but still high resolution) per-CPU timer AND probably
delivering that result to userland.

Read_tsc should only be used if either CONSTANT_TSC
or TSC_RELIABLE is true, so read_tsc is guaranteed
to be monotonically-strictly-increasing by hardware
(and enforced for CONSTANT_TSC by check_tsc_warp
at boot).

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

<Prev in Thread] Current Thread [Next in Thread>