Re: [Xen-devel] late lapic timer interrupts for hvm guest

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxxxxxxx>]

Keir Fraser wrote:
> On 28/10/08 07:45, "Juergen Gross" <juergen.gross@xxxxxxxxxxxxxxxxxxx>
> wrote:
>
> >      pt->scheduled = NOW() + period;
> >      /*
> >       * Offset LAPIC ticks from other timer ticks. Otherwise guests which use
> >       * LAPIC ticks for process accounting can see long sequences of process
> >       * ticks incorrectly accounted to interrupt processing.
> >       */
> >      if ( pt->source == PTSRC_lapic )
> >          pt->scheduled += period >> 1;
> >
> > We have ported our mainframe OS BS2000 to x86_64/XEN and we are using the
> > lapic timer as the main timer interrupt source (one shot mode). Finally I've
> > found the above coding which seems to be the root cause of late interrupts.
> >
> > I think the problem mentioned in the comment won't occur if the guest uses
> > the lapic timer in one shot mode with varying timer values (like BS2000). Or
> > am I wrong here?
> Right. That extra code could be conditional on setting up a periodic LAPIC
> timer.
Okay, here is the patch.

Juergen

--
Juergen Gross                             Principal Developer
IP SW OS6                      Telephone: +49 (0) 89 636 47950
Fujitsu Siemens Computers         e-mail: juergen.gross@xxxxxxxxxxxxxxxxxxx
Otto-Hahn-Ring 6                Internet: www.fujitsu-siemens.com
D-81739 Muenchen         Company details: www.fujitsu-siemens.com/imprint.html

When using lapic timer in one shot mode there is no need to delay timer
interrupts.

Signed-off-by: juergen.gross@xxxxxxxxxxxxxxxxxxx

# HG changeset patch
# User juergen.gross@xxxxxxxxxxxxxxxxxxx
# Date 1225183655 -3600
# Node ID 7640fba355fac233773d183077d7d01c8fd591f3
# Parent  874d0d673ecb55749cdc836942222ba684b365cb
don't delay lapic timer if one shot mode

diff -r 874d0d673ecb -r 7640fba355fa xen/arch/x86/hvm/vpt.c
--- a/xen/arch/x86/hvm/vpt.c    Thu Oct 23 15:38:52 2008 +0100
+++ b/xen/arch/x86/hvm/vpt.c    Tue Oct 28 09:47:35 2008 +0100
@@ -388,8 +388,9 @@
      * Offset LAPIC ticks from other timer ticks. Otherwise guests which use
      * LAPIC ticks for process accounting can see long sequences of process
      * ticks incorrectly accounted to interrupt processing.
+     * In one shot mode we assume this is no problem.
      */
-    if ( pt->source == PTSRC_lapic )
+    if ( period && (pt->source == PTSRC_lapic) )
         pt->scheduled += delta >> 1;
     pt->cb = cb;
     pt->priv = data;

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