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

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Message-ID: <4A4A6499.9000406@redhat.com>
Date:	Tue, 30 Jun 2009 22:16:41 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Gleb Natapov <gleb@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Sheng Yang <sheng@...ux.intel.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH v3] enable x2APIC without interrupt remapping under KVM

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?


-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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