RE: [Xen-devel] RE: Rather slow time of Pin in Windows with GPL PV driver

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

You have to be careful here. Xen will only ever deliver the evtchn interrupt to 
VCPU0. I can't immediately see anything preventing an HVM domain trying to bind 
and evtchn to another VCPU but you can see from the code in 
hvm_assert_evtchn_irq() that the guest will only be kicked for events bound to 
VCPU0 (is_hvm_pv_evtchn_vcpu() will only be true for Linux PVonHVM domains). 
Thus if you bind your DPC to a CPU other than zero and don't set it to 
HighImportance then it will not be immediately scheduled since default DPC 
importance is MediumImportance.

  Paul

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-devel-
> bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of James Harper
> Sent: 10 March 2011 06:27
> To: MaoXiaoyun
> Cc: xen devel
> Subject: [Xen-devel] RE: Rather slow time of Pin in Windows with GPL
> PV driver
> 
> >
> > It looks like KeSetTargetProcessorDpc(&xi->rx_dpc, 0) set the
> rx_dpc
> in VCPU0
> > only,
> > and in fact interrput for xennet are distributed across all VCPUS.
> >
> > By using IntFiltr from http://support.microsoft.com/kb/252867
> > to set interrupt affinity to VCPU0 only, without
> KeSetTargetProcessorDpc
> > commentted, we get quite stable ping time too., which is less than
> 1ms
> >
> > So I think this is the problem.
> >  KeSetTargetProcessorDpc should be discard.
> >
> 
> Ah. So when the cpu for the irq is different to the cpu for the dpc,
> you get the extra delay. That makes sense. It would also explain why
> XP didn't seem to see the same problem as I think the IRQ is
> directed to
> CPU0 there... I've been looking for the docs on what's different and
> can't find anything.
> 
> If you can confirm that you have no problems with removing
> KeSetTargetProcessorDpc I'll remove it, at least for >W2003 builds
> until I find the docs about what NDIS expects to do on what CPU.
> 
> James
> 
> 
> 
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