Re: [PATCH v3 1/3] Introduce a workqueue to deliver PIT timer interrupts.

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On Tue, Apr 17, 2012 at 03:00:10PM +0300, Avi Kivity wrote:
> On 04/17/2012 02:05 PM, Gleb Natapov wrote:
> > On Tue, Apr 17, 2012 at 01:42:31PM +0300, Avi Kivity wrote:
> > > On 04/17/2012 01:31 PM, Gleb Natapov wrote:
> > > > On Tue, Apr 17, 2012 at 01:29:04PM +0300, Avi Kivity wrote:
> > > > > On 04/17/2012 01:26 PM, Gleb Natapov wrote:
> > > > > > > It isn't, since you need to send an IPI.
> > > > > > > 
> > > > > > That is exactly what I forget whether you can send IPI from there :)
> > > > > > Anyway this is another reason.
> > > > > >
> > > > > 
> > > > > Actually I was wrong.  You can't smp_call_function_single() from irq
> > > > > context (deadlocks if two vcpus do that), but we send a reschedule
> > > > > interrupt.  So it should work.
> > > > > 
> > > > Ah, good point. So if we'll use irqsave versions of spinlocks we can
> > > > drop kthread?
> > > 
> > > Do we want 254 IPIs to be issued from irq context?  Could be slow.
> > > 
> > Where this number is coming from?
> 
> KVM_MAX_VCPUS.
> 
Ah, so you are worried about malicious guest configuring pit to
broadcast to all its vcpus.

> > > We can make the unicast case run from irq context and defer the
> > > multicast to a thread.
> > > 
> > We do not know if gsi is multicasted in the pit code though.
> >
> 
> We don't have to do it in the pit code.  The .set method can decide
> whether it wants a direct path or go through a thread (which thread?
> passed as a parameter?)
> 
Agree.

--
			Gleb.
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