Re: [PATCH v3] enable x2APIC without interrupt remapping under KVM

[Avi Kivity]

On 06/30/2009 10:36 PM, Eric W. Biederman wrote:
> > > The short version is I don't know what work arounds we will ultimately
> > > decide to deploy to work with real hardware.
> > >
> > > I have been seriously contemplating causing a cpu hot-unplug request
> > > to fail if we are in ioapic mode and we have irqs routed to the cpu
> > > that is being unplugged.
> > >
> > >        
> > Well, obviously we need to disassociate any irqs from such a cpu.  Could be done
> > from the kernel or only enforced by the kernel.
> >      
>
> Using the normal irq migration path we can move irqs off of a cpu reliably
> there just aren't any progress guarantees.
>    

Program the ioapic to the new cpu.  Wait a few milliseconds.  If it 
takes more than that to get an interrupt from the ioapic to the local 
apic, the machine has much bigger problems.

> > > Even with perfectly working hardware it is not possible in the general
> > > case to migrate an ioapic irq from one cpu to another outside of an
> > > interrupt handler without without risking dropping an interrupt.
> > >
> > >        
> > Can't you generate a spurious interrupt immediately after the migration?  An
> > extra interrupt shouldn't hurt.
> >      
>
> Nope.  The ioapics can't be told to send an interrupt.
>    

You can program the local apic ICR to generate an interrupt with the 
same vector.

> > > There is no general way to know you have seen the last interrupt
> > > floating around your system.  PCI ordering rules don't help because
> > > the ioapics can potentially take an out of band channel.
> > >
> > >        
> > Can you describe the problem scenario? an ioapic->lapic message delivered to a
> > dead cpu?
> >      
>
> Dropped irqs..  Driver hangs because it is waiting for an irq.  Hardware
> hangs because it is waiting for the cpu to process the irq.
>
> Potentially we get a level triggered irq that is never acked by
> the cpu that won't arm until the cpu send an ack, and we can't
> send an ack from another cpu.
>
>    

I think a spurious interrupt generated through the local apic solves 
that problem.  For level-triggered interrupts, mask them before 
offlining the cpu.

--
error compiling committee.c: too many arguments to function

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