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

Avi Kivity
2009-06-30T19:16:27Z

On 06/30/2009 10:08 PM, Eric W. Biederman wrote:
>> Can you elaborate? For kvm guests, the hardware is reasonably will implemented
>> and if not we will fix it. We need not cripple a feature just because some
>> hardware is broken.
>>
>
> 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.

> 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.

> 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?

--
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