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

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxxxxxxx>]

Hi,

When using lapic as timer source the hypervisor delivers timer interrupts
late.
In the source xen/arch/x86/hvm/vpt.c function create_periodic_time creates a
timer element with a "bonus" of 50% of the desired time until the interrupt:

    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?

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

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